Find a spark. A poem might start as a snippet of verse, maybe just a line or two that seems to come out of nowhere, and the remainder of the poem need only be written around it. Here are a few ways to generate sparks:
Play "Grand Theft Poetry." Gather a variety of books of poetry by different authors, or print 10 random poems off from the Internet. Then randomly pick a line out of each poem, trying to focus only on the first line you see instead of picking the "best" one. Write all these different lines down on a separate piece of paper, and try to arrange them into a coherent poem. The juxtaposition of two entirely different lines of poetry might give you an idea for your own poem.
Write down all the words and phrases that come to mind when you think of that idea. Allow yourself to put all your ideas into words.
It may sound difficult, but do not be afraid to voice your exact feelings in the poem. Emotions are what make poems, and if you lie about your emotions it can be easily sensed in the poem. Write them down as quickly as possible, and when you're done, go through the list and look for connections or certain items that get your creative juices flowing.
Try to fit into a particular scene you want to write about. For example, if you want to write about nature, try to visit a park or a small forest nearby. The natural scenery may inspire a few lines, even if they're not perfect.