to write any sort of poem u need to keep this points in your mind :
1.first take a topic on which u wanna write 
2.think about what u want to put inside the poem ( i mean the points u wanna highlight through your poem )
3.collect some rhyming words which are relative to your content 
4.keep some continuation or sort of link between the lines  
5.think about the rhyming scheme of your poem 
6.keep each line short and sweet 
and once u start writing a poem u automatically get a idea u can reffer to some poems written by someone else 
hope this helps u to write a short poem