In writing papers for literature classes, your thesis statement must take one side of a controversy. Simple summary or explication doesn't really constitute a thesis. Thus your thesis statement should focus on the way in which there are two possible ways of looking at the play.
The first way of looking at the play is one in which the play is understood as portraying Juliet as having found true love, and heroically doing everything possible to consummate it. In this vision, Juliet is a noble, selfless, and almost heroic character. The second way of looking at the play is seeing Juliet and Romeo as silly teenagers, infatuated with each other, and sacrificing their lives needlessly and imprudently.
To argue that Juliet has found true love, rather than just infatuation (or a "crush"), you can point out that unlike Romeo, she has not had previous relationships, and displays loyalty and constancy throughout the play. She also seems to have a more well-developed sense of responsibility than Romeo, with somewhat more mature concerns for the way this will impact other people.
A possible thesis statement would be:
Because, although young and somewhat naive, Juliet is steadfastly loyal to Romeo and willing to make great sacrifices to sustain the relationship, we should read this as an example of true love rather than just momentary infatuation.