Read other poets for inspiration. To write poetry well, you need to read it. Of course, don't just limit yourself to poems about nature. Explore as many types and genres of poetry as possible. This will help you understand technique, style and convention. Especially look into haiku, a style based upon syllables and nature themes.
Watch nature films. Traveling to far-off lands and mountainsides can be expensive and near impossible for some. Watch the Discovery Channel and the Travel Channel to see the sights you may never see otherwise. This could serve as inspiration for a future piece.
Wander about your backyard and take notes. Look for the things in nature that stand out to you. If you like birds, take notes on the way they fly or flock together or their colors. If it's flowers that inspire you, write about them, noting the smallest details for future reference.
Start working on your poem based on your experiences and studies. Write about the things that stand out to you, and use tools common to poetry such as metaphors and alliteration. If you want, you can even shape your poem to look like a plant or animal.
Revise and revise some more. To craft a truly great poem, make sure to edit it to perfection. Don't be afraid to cut things that both don't make sense and don't keep everything tightly knit. If a line hurts the poem and you can't fix it to your liking, it's perfectly fine to just yank it from the poem entirely.