First step is that you have to think about a topic next step is to make a mind map of it and then whichever topics you have written on the mind map just make a small poem of it 
A poem is basically a rhythmic plot of a story.
To write a poem we need a creative mind.
Before starting we need to think thoroughly about the topic u r going to write about
U need to think of a topic which can be easily and nicely written.
U need a dictionary open in front of u so that u can see the rhythmic words.
Hope this helps u