This is among my favorite novels and is the novel I think best serves as an introduction to reading Dickens. Just to add a few ideas to the excellent answer above, here's a few things to think about:
Very early in the novel, at the Christmas dinner, Uncle Pumblechook (what a great name for a character who is a bit of a jerk) tells Pip to especially "be grateful, boy, to them which brought you up by hand" (chapter 4). This is really a supreme example of Dickens's genius, because the reader has by now divined that Pip is an abused child, somewhat protected by Joe, but psychologically and physically abused by his sister. And the "by hand" expression, which Mrs. Joe brags of constantly, comes to mean specifically by beating you. So the reader sees this advice from Pumblechook, whom I have already described as a pompous rear portion of your anatomy (all right, I just said "jerk" earlier) as asking Pip to be grateful to those who are beating him! However, that is just irony; that is not the ingenious part. It is not long in the novel before Pip goes to visit Miss Havisham and meets Estella, and starting almost immediately, he becomes ashamed of his family's working class existence. This becomes infinitely worse once Pip receives news of his great expectations and expected fortune from a secret benefactor; he is now, as a young "gentleman," utterly ashamed of his home and family. THIS IS THE KEY TO THE NOVEL, WHAT I CALL "THE THEME OF PIP'S INGRATITUDE." It is the lesson Pip must learn in the course of the novel: to be grateful to those who brought him up--by hand or otherwise! We see the theme cemented in the latter portion of the novel when Pip is not grateful to his secret benefactor, and is indeed ashamed of him once he learns who that benefactor really is. And again, in the novel's final chapters, when Pip has gone bankrupt and is sick, it is good old Joe who nurses him back to health and even pays off his remaining debts. Before the novel is over Pip has indeed learned to be grateful to both his benefactor and to Joe and Biddy. It is worth noting how Biddy describes Mrs. Joe's dying moments:
And so she presently said "Joe" again, and once, "Pardon," and once "Pip." And so she never lifted her head up any more. . . . (chapter 35)
What Dickens has done, rather cleverly, is hide from the less observant by separating the words and capitalizing each of them that these last words of Mrs. Joe's form a sentence! "Joe, forgive Pip." The theme is that Joe is to forgive Pip for the sin of his ingratitude, for not writing, not visiting, being ashamed of his family.
As for Estella, I need only attempt to connect a few of the ideas presented above. Her name means "star" and she carries a candle while leading Pip through the dark rooms like a star in the darkness. In fact, Dickens wants us to get the star symbolism and so, when Mrs. Havisham tells Pip to "Call Estella" back to play cards with him, Pip describes her return down the dark hallway thus: "But, she answered at last, and her light came along the dark passage like a star" (chapter 8, emphasis added). And a few pages later, Pip sees Estella "pass among the extinguished fires, and ascend some light iron stairs, and go out by a gallery high overhead, as if she were going out into the sky" (chapter 8, emphasis added). Such repetition of an idea by an author of things they want us to notice is called "emphasis"--the author emphasizes what we are not to miss by repeating it.
So why connect Estella to a star? Well, stars are beautiful, and disregarding the fact that we know they are extremely hot, they appear to be cold twinkling up there in the night sky; they certainly provide no warmth to us located as far away as they are. And that is another part of the symbolism: stars are distant! So these are Estella's attributes: she is beautiful, cold, and distant. She can be admired but not reached, not touched. Other symbolism connected to Estella relates to Mrs. Havisham's jewels, which are also cold and hard, beautiful and unfeeling. But the more solid case is to be made for the star connection.
What matters about this entire answer is that I encourage you to learn to see the details, see what is repeated, notice the "be grateful" idea and not merely chuckle at the "by hand" joke, but see the fact of Pip's ingratitude. Notice the repetition of ideas connected to stars with Estella and try to put them together. Someone once said that the best things in books are not shouted out by the author, but whispered between the lines. How many readers, do you think, read the account by Biddy of Mrs. Joe's death and last words and never put the sentence "Joe, forgive Pip" together or realize the import of those words in relation to the main theme of the novel?
P.S. I cannot conclude without pointing out that Dickens was pressured by friends not to end the novel with Pip and Estella parting. He had originally composed an ending where they meet very briefly, and Pip tells us she is married to someone else, but his friends asked him not to use this ending and so he did not. So although it is true that Estella says, after Pip states they are friends, "And will continue friends apart" (on the last page of the novel), Dickens does not expect us to believe that; thus the last words of the novel are "I saw no shadow of another parting from her." The implication is that Dickens has crafted a happy ending, under pressure from his friends, and that Pip will in fact marry Estella. Just before describing seeing her again, Pip mentions the death of her husband and remarks, "for anything I knew, she was married again." So the fact is, she's not married again, and Pip has no intentions of letting her go again.