I need to write a poem for my English class. The thing is, I don't know how to write a good poem. I've read loads of poems, but I just don't know how to write a good poem.

Answer 1

Here are some approaches for writing poetry.

http://www.poetrymagic.co.uk/approaches.html

http://www.poewar.com/poetry-writing-tips/

A couple of suggestions : 1) remember that poetry does not have to rhyme or have meter... Free verse works!
2) Chose a topic you have strong feelings about... it doesn't have to be serious feelings, it can be humor.
3) Take a piece of paper and jot down Words you think of when you think of that topic NOT sentences, just words.
Then you can organize those words into your poem.

Answer 2

You received good advice from GuruBlue. HEre's an example of using that advice:
Topic--Barbie Dolls
Words to use--Barbie, cute, dresses, careers, fun
Poem from those words--
Barbie is fun
She is cute and wears neat dresses
Barbie shows me I can choose
I can choose a career
I can choose to have a fun career
I can choose to wear neat clothes, too.

This isn't a "good" poem, but it does fill the assignment parameters.

You can also take a word and make a poem from that. Example:

BARBIE

B is for the best she can be
A is for all the careers she does
R is for the really neat stuff she can do
B is for bringing me ideas
I is for what I can do now
E is for everything she has shown me.