Shakespeare's best tool for creating suspense and tension is that fact that he tells you in the prologue what is going to happen to the "star-crossed lovers". We hope against all hope that they will find a way to get around the prophesy, but to no avail.
First, there is the tension between the two families. It begins with thumb-biting and quarreling in public which is quelled by the Prince.
We have tension in Romeo's sullen and moody rejection by Rosaline, and the attempt to cheer him up by taking him into the Capulets' household for a party. Woe until him if he is caught in the enemy's house!
There is the developement of a love affair between the two offspring of the town enemies. Lots of speeches about how in the world can I love someone I hate...internal conflict and tension there.
The Friar secretly marries them...again the suspense of getting caught...and then everything is mucked up when Tybalt kills Mercutio who is then avenged by Romeo. Romeo, newly married, is banished as the murderer of his new kinsman.
The tension is really thick when Juliet is expected to marry Paris when she's already secretly married. She and Friar come up with this amazing plan which fails because the letter is not delivered in time and Romeo gets the news from someone who is not privy to the secret plan.
Romeo goes to buy a quick poison and kills himself minutes before Juliet awakens.
If that's not enough evidence of tension, I am baffled.
Shakespeare creates tension through several "oppositions" -- that of the two families Capulet versus Montague, that of the cousins Tybalt and Romeo, that of the Prince (government) versus the kids/gang in Verona. (Romeo, Mercutio, etc.) Look for language that show opposition between the two characters. Good luck!
Okay! If you are struggling you might want to reread Romeo and Juliet, look at notes or anything you have from class, and maybe even do research on Romeo and Juliet. You know just look up information on it. This website would be a very good place to start. If you need help let me know!