Determine the context of the phrase. Are you planning to say it to a girl you like? Your grandmother? Your sportscar? This will help you establish how to word the phrase.
Say "tu es tres belle (beau for a man)" if you mean to use the most common form of "you are beautiful." It is pronounced "Too ess tray bell." Or "Too ess tray bo."
Saying "tu es jolie" is also a common compliment in France, which translates to "You are pretty/attractive/good-looking." Jolie is pronounced like Angelina Jolie's last name, except the "j" has more of a "shhh" sound to it, like "Je t'aime" ("I love you) does.
Say "Vous êtes belle" as the more formal way to say "You are beautiful." It is pronounced "Voo-zet bell." This phrase is particularly useful if you are addressing someone you do not already know.