LOL. There is always the "Skinny B#$ch", which is diet coke and vodka, this usually has about 100 calories. You could also do vanilla vodka, Captain or coconut rum (surprisingly good) with diet coke. I also like Raspberry vodka and sugar free Red Bull. My personal favorites are Cosmos and Mojitos, which have about 200 calories each, just watch out for super-sized martini glasses, a Cosmo is only suppose to be 4 oz. Wine, Champagne and light beer are also good ideas.
Stay away from high calorie mixers like tonic water, regular soda, creamy drinks, frozen drinks and "fruit juice" (which is really just mostly sugar when you get it at a bar and not 100% juice).
If you are going to pre-game at home, you have way more choices with diet soda, Cristal Light, Light Cranberry juice, etc. Sadly enough, I did come up with "better for you" recipes for frozen mudslides and frozen Bellini's, but I wouldn't call them low cal
Have fun, be careful, and don't try to "save" calories for the alcohol by not eating all day...drinking on an empty stomach is a really bad idea. If you can, try to get some cardio in the day of and after the partying to help counter the extra calories. Just don't get into the bad habbit that I did when I was your age and be "good" during the week and live it up on the weekends, it really does not work very well.