All the suggestions given in Resources for adults working on their own apply, since one of the best ways to improve your writing is to make a habit of going deep into good poetry written by others. I'm adding here only a few items that are especially helpful for beginning poets:   Some Useful Books on How to Write Poetry Ted Kooser, The Poetry Home Repair Manual. Highly recommended for a sensible, down-to-earth approach to the basics, and once in a while it catches fire, as in his discussion of metaphor.
John Fox, Finding What You Didn’t Lose: Expressing Your Truth and Creativity Through Poem-Making. John Fox writes from the perspective of a poetry therapist; his second book is in fact called Poetic Medicine. Here he gives all sorts of ideas for getting started, as well as good suggestions for first steps at refining your work. Juicy quotations and poems.
Frances Mayes, The Discovery of Poetry: A Field Guide to Reading and Writing Poems, 2001. A systematic approach, aimed at complete beginners. Emphasis on basic terminology, and useful for its explanations of that.