Words Words Words
A problem in Shakespeare is not the words you don't know, because you know you don't know them—if you run across fustian, you look it up. More insidious are the words you think you know and thus you don't look them up, such as nice (which actually means foolish, ignorant, or even lascivious) and want (lack) and mere (total, unadulterated, as in swearing that we are mere usurpers, tyrants). The resources below help you muddle through with confidence. (The ISC Press Readers' Editions call out these changed words to you as you're reading.)
The OED, the Oxford English Dictionary, is “the definitive record of the English language." The OED defines every word from the beginning of recorded English, and you can see how it changed along the way.
If you belong to a university in any way, you might have access to the OED online; as an individual, you can subscribe for $300/year (they no longer publish the full print version, which was twenty volumes). Otherwise, your local library might subscribe and if so, you can get a library number with which to access the OED. Check the libraries in cities larger than your own—it is worth a trip to get a library card and thus access to the OED.
Alexander Schmidt's 1902 Shakespeare Lexicon and Quotation Dictionary is the next best thing to the OED. You can buy it in a two-volume set, as shown here.
O use the FREE online version at Tufts University: Shakespeare Lexicon.
David Crystal and his son Ben have spent many years researching the words of Shakespeare, and their web site is an enormous resource: ShakespearesWords.com.
If you prefer a book, that's available as well:
There are a number of books on how to pronounce Shakespearean words, most of which are names and places and mythological beings. This is our favorite one, All the Words on Stage: A Complete Pronunciation Dictionary for the Plays of William Shakespeare.
Original pronunciation, or OP, is a recent study that explores how Shakespeare's language sounded in the Elizabethan era. David Crystal and his son Ben Crystal are the major proponents of this study. They base their findings on what other writers said at the time about the pronunciation of words, on original or variant spellings that give clues (the town of Calais is spelled Calice in the First Folio), and on rhymes in the plays (love rhymes with prove).
The Crystals have produced abundant material for those who want to look into OP—books, web sites, recordings, and more. Here is some info, and you'll find much more at their site.