My teacher assigned us to write a poem, but no matter how hard I tried I have no idea what to write about. What should i write about and how to get inspiration?

What I normally do when I need inspiration for anything is I do a free-association exercise. Basically I'll go sit somewhere different from where I normally work.. maybe outside.. in a different room... etc. Then I let my mind wander and write down random thoughts.. things I see (this work especially well if you go sit in a public place and people-watch). Sometimes just taking yourself to a different location and letting yourself daydream will do the trick.

Think about all the things you love, like your dog or a certain book or nature. Think of the way it makes you feel. Or write about something you see on the news. Or you can simply write down random words and phrases and see what sticks.
Does it need to be a specific format?

Try doing it in the morning when you've just woken up, or before you're completely asleep, but halfway there.

^ If you remember your dreams, this can be a great start. Keep a journal right beside your bed. Then when you wake up in the morning, quickly jot down a short description of your dreams. That can be a jumping point for your writing, or at least provide a touch of inspiration.