The links in the above answers are certainly good examples of different family crests, but remember, Shakespeare's Capulets and Montagues were fictional families. The above examples are likely links to actual Capulet and Montague family crests.
If your assignment is to design a family crest for either the Capulets or the Montagues, likely, your teacher is looking for some creativity, both in art and in thought. I encourage you to consider some key identifying traits of the Capulet and Montague families. Then, seek some common crest symbols that represent those characteristics. Most family crests also have a saying somewhere on them, which you could make up just as easily as look up. Finally, keep in mind that Shakespeare was English. When searching for different ideas, if you wish to keep your product as authentic as possible, try to stick with examples of other English family crests.