Decide what to write about (i.e. love, death, pain, sorrow). Some people can start writing poems without really deciding what to write about; when their pen or pencil hits the paper the words just flow. Most people, on the other hand, need a topic and as a beginner, this is the best way to start.
If you are describing something (i.e. a bird), write down descriptive words about it (i.e. looks, colours), the emotions it evokes and the type of story you would like to tell about it (i.e. the bird fell out of a tree). If you are writing about an event in your life, write down your feelings, thoughts and reactions to that event. Basically, you are making a list of all the components you would like to include in your poem.
Decide what style of voice to use. Will your poem be written in first person or third person style? Will it be narrative or descriptive?
Select a style of poetry. There are many different types of poetry to choose from. If you've never written poetry before, choose an easy style like free-verse or rhyming. If you have written poems before, continue with the style that works for you. Once you have more practice, you can then start experimenting with other styles of poetry.
Write. Whatever comes to mind. Don't worry too much about style right now. The first thing you need is words; fitting it into your chosen style will come later.
Read your poem and compare it to your chosen style. If it fits, great. If not, edit your poem to suit the style you've chosen. For example, if you picked a rhyming style, make sure you've stayed with that same format throughout the poem (i.e. abab; The wind blows from the west, and I've decided many times, to lay my body down and take a rest, and write this poem so it rhymes!). If some words don't rhyme, use a thesaurus or go to rhyming websites to find the appropriate word.
Re-read, edit and then re-read your poem again. Also, read it out loud. This is the final step and you may end up repeating it. That's okay. It takes time to finalize your poem.