You also need to give some thought to the form of poetry that you want to write.
Do you want to write a 14-line sonnet that has a strict pattern of syllables-per-line and a carefully controlled rhyme scheme? Or can your thought be expressed in a much shorter form, such as haiku, or limerick? Or do you just want to let your thoughts flow in free verse?
These questions are more than just technical. The form of your poem should be in tune with the content of your poem. If your thoughts are more or less logical, you might want to try a sonnet; if your thoughts are random and highly emotional, then free verse is probably the appropriate form for them. If you want to express one simple combination of image and emotion, then a haiku would be appropriate.
You also need to consider the kind of language that you want to use. Some poems are written in standard, "proper" English; others are written in slang, or with invented words. Again, it all depends on the message that you want your poem to deliver.
I didn't know that there were steps to composing a poem. In many ways poetry springs, as a new creation, from the mind and from the heart.
I see poetry as a sort of shorthand of the soul. Once you have something to say (and that's the real challenge, isn't it?... actually having something you want to say) work hard at expressing it in as few words as possible. Make every syllable count, make every sound mean as much as you can.
There is an excellent book called Sound and Sense written by Laurence Perrine. In a way, the title says it all: poetry is about sound (the sounds and rhythm of words) and sense (the sense those words make to the reader). I would suggest if you are really interested in writing poetry and learning more about the art and craft of this noble endeavor, get a hold of the book and read it.
I often think of poetry as good old common sense wrapped up in a brand new, original package. Start to collect ideas and strive diligently to present them in your own, individual way.
The first thing you are going to need when writing a poem is something to write about. Poetry is an intense condensing of an emotion or an experience, and without an experience, emotion or thought to write about, you're not going to have much luck. To find something to write about, pay attention to the world around you. What strikes you as particularly beautiful? Does anything happen to you that makes you feel strongly? A poem can be about an entire experience, or just an image as simple as the sun glittering on the snow. Anything that moves you can be good material for a poem.
Once you have something to write about, the next step is to actually write some thoughts down. I recommend doing this in several different ways. The first thing you need to do is boil this topic down to its most important feeling. If you are writing about how beautiful a sunset is, what is the most important message that you want to convey? Do you simply want to describe the sunset? Or describe how it made you feel peace in a hard time in your life? Or did it strike you how beautiful it was over the landscape of a dirty, busy city? Deciding on your focus will help you to take your poem in the right direction.
First, write down any imagery involved in your topic, meaning, any of the five senses that are involved. For example, if you are writing about a sunset, describe the sights, sounds, tastes, touches and smells that are involved in that experience. Once you have the five senses down, I would recommend trying to come up with some comparisons. For example, if you are describing how pink the sunset is, what could you compare it to--cotton candy? A girl's dress? Bubble gum? Lip gloss? Finding objects out there in the world to compare to what you are trying to describe will add more depth to it.
Once you have your focus, imagery, and comparisons, then put it all together. Take all of your brainstorms, and piece the phrases together in a way that makes sense. Writing a poem can be a lot easier if you do several brainstorming activities first, and then use those to pull your lines from. That way, you have great poetic lines already, and actually writing the poem will just be a matter of patching them together. I hope that helped; good luck!