The most important element of your introduction needs to be the clear establishment of your THESIS STATEMENT.  You need to let your reader know what exactly you are going to prove about Myrtle Wilson.  Remember that a good thesis statement is going to be argumentative, so that means that you need to be making a claim about Myrtle that someone else, conceivably, could disagree with you about.  You can't argue that Myrtle is a large woman who Tom is having an affair with.  You can craft an argument about why someone like Tom would be drawn to someone like Myrtle. Different readers might have different opinions on this.  You can't argue that she mistook the car and was hit on the road.  You can argue that Myrtle is unrealistic in her understanding of Tom's feelings for her.  

There are all kinds of ways to introduce your topic, but a solid and straightforward approach would be a "funnel" introduction.  Think of a funnel -- broad at the top, but narrow at the bottom.  Your introduction could talk about the TOPIC of the thesis broadly, about "life in general" kind of information first, then narrow a bit to talk about the topic in general as it is seen in the novel, and then most specifically how it relates to Myrtle.  That last sentence is your thesis statement about Myrtle. So if you where going to write about the example thesis from above: your opening could be about true love and relationships in general; the middle of the paragraph would talk about the love and relationships we see in the novel; and the end of the paragraph would state what I said above about Myrtle being unrealistic in her hopes of this relationship. 

Remember that your introduction can be more than one paragraph long, but that by the end of it, the reader should have a clear understanding of what you are trying to prove.  This will help your reader as they go on into the body paragraphs which will provide all of your textual evidence and analysis.