but to me these are people who realize that they have made a tremendous mistake but don’t want to admit it or face up to each other. I don’t mean the baby, but their lives.

So a thesis statement about symbolism in the story would need to account for this. I guess another way of thinking about a thesis would be to ask some questions: How does the setting of the story, or any of the descriptive details Hemingway provides, relate to this theme? Do these things contrast with this feeling of disconnectedness or reinforce it? Can these details be seen as ironic in any way? I think if you consider these questions, you can come up with a good thesis about how Hemingway uses the landscape as a way to comment on, and distance himself from, these characters.

Another way to approach a thesis looking at symbolism in "Hills Like White Elephants" is to identify the meaning behind the story and the meaning behind the symbolism used in the story in your thesis statement. 

What is your interpretation of the story's meaning? What is your interpretation of the meaning behind the symbols used? Is there a unified significance that connects each item/example of symbolism?

This is often seen as a story with a single issue at its heart, one that subtly animates a philosophical and social dilemma. The decision as to whether or not Jig should abort her pregnancy is connected to a question of modern decadence and, also, to a sense of purposeless wandering. At the train station, the couple is also at a cross-roads. 

"The girl calls attention to the symbolic value of the setting and indicates that in choosing to have an abortion and to continue to drift through life they are choosing emotional and spiritual desiccation" (eNotes). 

A thesis acknowledging the meaning of the story then would include ideas of choice/turning-points, binary contrasts (to indulge in aimless decadence or not, etc.) or conflicting visions of the world. The symbols of the text match up well to these ideas and can be said to articulate the story's meaning almost as much as the action and dialog do. 

How about this: "Hemingway's classic story "The Hills Like White Elephants" appears to be simply a realistic description, but is in fact highly symbolic"? That would allow you to make any argument you wanted about the nature of the symbolism.

If you wanted a more specific argument, how about this: "Symbols found in both dialogue and description in Hemingway's "The Hills Like White Elephants" convey the deepest meaning of the story"? That allows you to be highly specific.

A story, however short, is seldom written to demonstrate a thesis in the same way as an academic essay, so there may be a variety of plausible answers to this, based on the reaction of individual readers and what they think is the story's most important theme. With that in mind, here is my view:

A life lived purely for selfish pleasure is ultimately hollow and boring, deracinating the one who lives it and cutting him or her off from anything that might give life meaning.

This sounds dangerously like a moral, which is why Hemingway never says any such thing, but he is constantly demonstrating it. The lives of the two protagonists are superficially glamorous and exciting. As the girl says, they "look at things and try new drinks." The flatness of their conversation, however, reveals that they are sick of everything they do. Even the revelation that they are having a child, rather than being the traditional source of joy and celebration of a new life, merely entails a clinical discussion on how to get rid of the encumbrance most efficiently.

The girl's dissatisfaction with this lifestyle is shown by her reluctance to abort her child, in her attempts to find color and excitement in life, even in her extensive remarks on their surroundings. The man is too steeped in selfishness even to make any attempt or pretend to care about such things. The irony is that the only effort he makes is to persuade the girl to end a new life so he can continue his existence undisturbed, but this existence he protects is entirely pointless.

Nihilism is a constant theme in Hemingway's work, and it often appears that he was only saved from it himself by a strong belief in his own writing. Characters like the man in the story often seem to be Hemingway's sketches of the type of person he might easily have become himself if he had not been a writer.

A thesis statement for a short story establishes the theme and tone of the text that follows, and expresses a conclusive point that the text will presumably validate.  A thesis statement for Ernest Hemingway’s short story Hills Like White Elephants could read something like this: ‘In his story Hills Like White Elephants, Ernest Hemingway illuminates the fragility of and emotional emptiness at the center of a relationship that is threatened by the interjection of an unborn child.’

Hills Like White Elephants depicts a man and a woman obviously engaged in a romantic relationship that is just as obviously undergoing serious strain.  Evidence of the tensions permeating this relationship is presented at the story’s outset, as the couple await the arrival of a train and struggle to pass the time in conversation.  Hoping perhaps to break the ice, the woman observes the hills off in the distance:

“They look like white elephants,” she said.
“I’ve never seen one,” the man drank his beer.
“No, you wouldn’t have.”
“I might have,” the man said. “Just because you say I wouldn’t have doesn’t prove anything.”

This opening exchange reveals a relationship in crisis.  What is only gradually revealed, however, is the immediate cause of that crisis – the woman’s pregnancy.  We are not told, of course, that the topic of conversation is the couple’s decision to abort the pregnancy, but it’s not difficult to figure out.  In the following exchange, it becomes apparent that the man is more enthusiastic about subjecting his girlfriend to a surgical procedure than is the woman, and it’s also clear that the result of this operation will presumably repair what is damaged in their relationship:

'It's really an awfully simple operation, Jig,' the man said. 'It's not really an operation at all.'
                The girl looked at the ground the table legs rested on.
                'I know you wouldn't mind it, Jig. It's really not anything. It's just to let the air in.'
                The girl did not say anything.
                'I'll go with you and I'll stay with you all the time. They just let the air in and then it's all perfectly natural.'
                'Then what will we do afterwards?'
'We'll be fine afterwards. Just like we were before.'
                'What makes you think so?'
                'That's the only thing that bothers us. It's the only thing that's made us unhappy.'

That the man is the principle advocate of the abortion-as-resolution-of-problem position is repeatedly emphasized, as in the following continuation of this exchange:

'Well,' the man said, 'if you don't want to you don't have to. I wouldn't have you do it if you didn't want to. But I know it's perfectly simple.'
                'And you really want to?'
                'I think it's the best thing to do. But I don't want you to do it if you don't really want to.'

Hemingway’s couple pretends to be conflicted regarding the effects a child will have on an otherwise loving, mutually-supportive relationship, but the reality appears far different.  The strained tones and the pretensions to an idyllic existence that once existed create an ominous tone.  The discussion about whether to go through with the abortion reveals underlying fissures in their relationship that they refuse to openly acknowledge.  The “unwanted” pregnancy is only the immediate or near-term cause of tensions between the man and woman; the longer-term, underlying cause – the ‘elephant in the room’ if one wants to be quaint – is the fact of a relationship seemingly built on superficial attractions that conceals the absence of a deeper emotional commitment.  This couple fears that a child will ruin their relationship because they will no longer be free to live the carefree existence they have ostensibly enjoyed to date.  In an exchange toward the end of the story, the woman seeks solace in the liberating consequence of the abortion only to have the man dampen those expectations despite his advocacy of her having the abortion.  The man has employed a passive-aggressive approach to urging the woman, Jig, to go through with the procedure, subtly moving the action in his desired direction while attempting to place the burden of the decision on her. This is not a healthy relationship irrespective of the issue of the woman’s pregnancy, and a thesis statement on Hemingway’s story should advance that proposition.

It appears that you have been given a choice of elements to analyze. So, for instance, regarding style-- Hemingway's story has long been used as an example of symbolic literature.  In addition to this stylistic tool, another that you can discuss is the use of double images to convey the ambivalence of decision that the young man and woman have regarding her having an abortion.

If you wish to deal with theme, perhaps you can examine one that is prevalent with Hemingway:  One cannot return to a former state after a momentous incident in one's life happens. The young man tells his girlfriend that everything will go back to the way it has been if she has the abortion: "They just let the air in, and then it's all perfectly normal."  Even so, the young woman is not sure that she can or even wants to return to their former life of merely "looking at things, and trying new drinks."

Go to the sites listed below for information/analysis of the story as well as how-to's on essay writing.

I hope this helps and good luck!

In order to come up with a thesis for Hemingway's "Hills Like White Elephants," you first need to determine what the story is about and what message Hemingway is trying to convey.

The story is about a couple waiting for a train at a train station. They are talking about the woman's pregnancy. The man wants her to get an abortion. She does not want to. There are clues this conflict is putting a great strain on their relationship, and probably reflects strains that were already there before the pregnancy brought the relationship to a crisis point.

With Hemingway, however, the problem is that he offers no explanation of what is going on. For example, he uses very plain words, such as "said," which don't offer information about how the couple is speaking to each other. So how do we know the couple is under a great deal of strain, angry, and saying the opposite of what they mean?

One way is repetition. For example, does the man repeat variations of "I wouldn't have you do it if you didn't want to" so often that he starts to sounds insincere?

Another way is through the bitter responses they give to what each other is saying. For example, when the man says, "Oh, cut it out," or when the girl says, "Can't we maybe stop talking," we might sense hostility.

A third way is symbolism: is sitting in a train station going nowhere symbolic of the relationship going nowhere?

If these seem to you like three signs of a dysfunctional, strained relationship, a thesis could be encompass how Hemingway reveals the unhappiness in this relationship through repetition, diction, and symbolism.