One of the first steps to learning to write poetry is to read the poems of others. A good way to begin is to start with the classics, and work your way up to contemporary poets. You may consider taking a course in poetry if you prefer a more structured environment, or embark on your own poetry course. The goal of reading as much poetry as possible is to find what kind of poetry appeals to you the most and the kind of poetry you'd like to write yourself.
After you've read as much poetry as you can, the next step is to increase your word power. Some of this will be accomplished by reading a lot, but there are also resources available to help you build your vocabulary, such as word-a-day websites and emails that will help you learn different words and how to use them.
Although there are several different kinds of poetry, almost all poetry uses metaphors of some kind. A lot of poetry also heavily uses imagery. Start combining metaphor and imagery in your poetry to find styles that you like, and that convey the essential meaning of the poem or the emotion or message that you are trying to communicate.