Start by reading, reading, reading. If you read enough and find some things that you like in them, then you can start using those things yourself. As T.S. Eliot said: "Immature poets imitate; mature poets steal; bad poets deface what they take, and good poets make it into something better, or at least something different." So, imitate first (I highly recommend you look into Eliot, one of the best poets ever, especially "The Lovesong of J. Alfred Prufrock" "The Waste Land" "The Hollow Men" "Rhapsody on a Windy Night" as some of his best work though it may be tough). You could always look into literary theory but reading is the best. Take Faulkner's advice: "Read, read, read, read everything, trash, classics, everything. And then write. If it is good keep it, if not, throw it out the window."
Keep an open mind, don't just consider literature. Think about math, about the anatomy of frogs, the Keynesian model of short-run aggregate supply. Think in images. A poem conveys an image, a moment in time. Look to your life and picture it not in words but in a whole string of images that make sense only in images and then add the best words you can to them. Understanding a poem comes from understanding this. Poems are about life, so live and create. Passion, feel, let things hit you in the face and then strike them down on paper. You really have to suffer to write great poetry, suffer in the creative spirit. Suffering will let you understand the influences that have led to the composition of the greatest poems of all time. Be serious, let art come through you, don't hold it back with shallow emotions.
If you want a place where people can review your poetry, go on to poetrycritical.net and make a free account. Its a place where people can comment and rate your poetry and help you out. You can check out some pretty good poetry that is not mainstream on there as well. P.S. if you do use this site, you should take a look at my poem "Berceuse." I will try to look at yours as well if you post any. Hope this helps.