There are many resources a student can find for help with understanding the Shakespearean language. To begin with, eNotes actually offers an enhanced e-version of all of Shakespeare's plays, including Romeo and Juliet, with an embedded glossary. Within the e-text versions, you'll see different words underlined in red. If you hover your mouse over the underlined word, you see the modern understanding of the Shakespearean term. In addition, the e-text provides you with the original Shakespearean text on the left-hand side and the modern translation on the right-hand side. For example, the eNotes e-text version is very helpful in informing us that in the line in which Romeo says of the moon, "Her vestal livery is but sick and green," in the famous balcony scene, the word "vestal" means virginal, or symbolizes chastity while "livery" refers to a dress, or the way that someone dresses (eNotes, II.ii.8).
Aside from e-Notes enhanced e-text versions, you can also find quite a few scholarly websites that offer Shakespearean glossaries. Towson University offers a glossary for the entire Romeo and Juliet play ("Romeo and Juliet: A Glossary"). The glossary shows the line the Shakespearean word, term, or phrase is found in; the original word, term, or phrase; as well as the modern translation. Other glossaries can be found on-line in which you can look up individual Shakespearean words (Shakespearewords.com).
Also, if you prefer something in print, many publishing companies release the same sort of thing that eNotes offers as an e-text, a version of the play with the original Shakespearean language on the left-hand side and the modern translation on the right-hand side, plus additional glossary entries. For example, the publisher Barron's Educational Series, Inc. produces the series Shakespeare Made Easy.