Pick an event that affected you one way or another.
Write about that physical event. (Includes brainstorming every last detail you can remember) If you find rhyming comes easy once you have the guts of it out, then rhyme. It isn't required. Most confessional poets don't rhyme.
Just remember to write about an experience(and how you behaved during it), and if it is a good poem then people will understand what you were feeling.