Aw...my daughter just turned 16 this year too. We were supposed to have an Alice in Wonderland party, but we moved instead.
So I still owe her a party. But I agree, a theme makes it easiest to plan.
Some things to think of first: Budget??? Do you want a large party? Or a smaller party with closer friends? Family coming? Do you want to have it at home--or somewhere else? Do you want formal--or casual? Do you want it to be more of a dinner-type thing, or a dance all night long type of thing? Throw me some hints. I looooove parties!