I was thinking of doing a story like two best friends and one best friend just totally changes and ignores you and goes off with someone else. Whenever they go out she like rubs your face in it. Don't know where you belong, etc. I just need help with a plan and story base. Any suggestions and ideas will be appreciated :D
If your assignment can be written as a story, this direction does afford the opportunity to employ dialogue which certainly enlives a narrative. However, if a narrative essay is the assignment, then do not use dialogue; instead, follow the regular format of an essay but narrate a sequence of events that contain narrative examples that lead to the outcome which illustrates your main idea, or thesis.
One example of an excellent narrative essay in George Orwell's "On Shooting and Elephant" which is usually anthologized in 12th grade literature books if you can get a hold of one. Also look for Emerson's and Thoreau's essays which are great examples.
You call this a composition, but it sounds more like an interesting story to me. A good story generally has dialogue (conversation) as well as description. Effective dialogue is sometimes difficult to write, so I'd suggest simply making it sound as realistic as possible without getting too casual or making it too stilted. Aim to write the words in the way your characters would speak--which would be the way you speak, I presume, since they're teenagers. Best of luck!
It sounds like you already have a good plan of attack for your composition. Hopefully it is one that is based on a real life experience (though not too painful, I hope). Arrange your thoughts and come up with a good, catchy opening sentence that will draw your audience's interest. You should probably plan for at least one paragraph to describe your prior relationship, followed by at least one paragraph that illustrates the changes. A final paragraph (or more) should probably describe your present situation. Be sure to think about how you want to conclude your story. You probably don't need to use your "that was the last straw" statement at the end; in fact, it may be more appropriate to use in your paragraph about the cause of your breakup. Good luck, and I hope this helps.