This is an important question. Whenever you work on a paper, you need to have sources. In other words, it is important to ask how you know what you know. That said there are a few quick and effective ways to find sources.
The easiest place to go is enotes.com. So, you have taken the first step. If you did not know, there are lot of study guides on Shakespeare's Romeo and Juliet here. I will list three of them for you. Once you read these, they will give you other sources to track down at your library.
If you want more information, you can go to online journals. More and more journals are being digitized now. For example, you can go to Project Muse or JSTOR. These services have hundreds of journals. The great thing about digital journals is that you can do key word searches. For example, you might search "Romeo" and "characterization" as key terms. A good research library should have these subscriptions.
Another way to find sources is to get a recent book on the play and look through the footnotes and bibliography. Good scholars will be abreast of what is out there in terms of scholarly material.