A wide variety of dictionaries is available to you online or at a nearby bookstore. You can buy a tiny travel dictionary with everyday phrases or a thick reference book of medical terminology. Do you need a portable paperback dictionary to throw in your backpack, or do you need an exhaustive dictionary for home reference? Will you be using your dictionary primarily for looking up conversational phrases on the fly, or at your computer writing an essay every time you use it? Do you just need a thesaurus, or would you prefer a book with extensive indexes and grammar explanations? Do you need a certain regional dialect or any specific subject matter?
You can also eschew printed documents altogether and use online resources such as www.spanishdict.com or dictionary.reference.com/translate.
Another option is to use an all-Spanish diccionario. That seems todefeat the purpose of having a dictionary in the first place, doesn't it? Actually, using an all-Spanish dictionary can help you internalize the language in a way that memorizing translated English definitions cannot.
You'll need to start small. Begin with a children's dictionary, almost like a "My First Words" book that parents might buy for their child. An excellent choice is the Vox Diccionario de Primaria de la Lengua Española by NTC Publishing Group (ISBN 0-658-00066-7).
Picture books in Spanish can also serve as mini-dictionaries. Visit your local library and see if you can find Barron's Educational Series Descubro mi mundo with books like Los Sentidos and Los Elementos.