How would I write a descriptive poem about myself?

I would write down what you want to say about yourself, then go from there. Don't worry, this is not a 'plan'. For example hair colour, eye colour, interests, etc. Then relate those to things you wouldn't normally think of. For instance, I have curly brown hair, so I might call them 'wisps of a chestnut breeze...' By comparing yourself to other things, you are naturally creating metaphors because metaphors are comparisons, and if you compare yourself to something using 'like' or 'as', you have created a simile.