Sounds like you'll be doing a "Men are from Mars, Women are from Venus"-type essay, eh? Let's see, you might introduce your topic with an example of a misunderstanding in a relationship ("Jon gave Clarisse a leaf-blower for her birthday and couldn't understand why she hit him over the head with it. She'd been complaining about the leaves in the yard, after all." That sort of thing.) Then you could use the body of the paper to outline some of the most common types of assumptions about male and female relationship behaviors, and why the other gender finds them incomprehensible. Then conclude with a paragraph that restates your thesis and sums up the points you've made.
Do you have any resources to find out what the behaviors are that you'll be talking about? You'll undoubtedly be able to find relationship books in your local library, and the Web should have lots of information, since this is a popular topic. You can use your own experiences, of course, but it's always good to back them up with evidence from people who are experts in the field of relationships.
I hope this advice helps get you started. Just keep your topic in mind as you write each paragraph, and remember that you want to keep your reader interested, so use material that you find interesting. It's a great topic--I wish you the best of luck as you write!