Im turning 15 soon.What should we do at my party?!

For the invitations you could send out some fun mask invitations. Put all of your party details on a piece of card stock, attach it to a metallic mask that is decorated with sequins, jewels, feathers, etc., and hand-deliver your invites. If its a costume party you can also add a little saying such as "Find a mask and wear it well so your true identity no one can tell.
Use xmas lights to decorate plants/ trees doorways, windows ect.. you can hang glittery fabric over doorways or walls. You can also swag them across your ceiling. Spray paint silver or gold (depending on which color you select) whatever you can to add additional glitz to your party room. For example, disposable serving dishes (just be sure to place a doily in the center of the dish so the food doesn't touch the paint), old chairs, candle holders, etc. Hang strands of balloons across your ceiling, along with creating bouquets with trailing curling ribbons to place throughout the party room.
Twist streamers in your party colors throughout the party room. You could even cover your entire ceiling with colorful twisted streamers. Scatter confetti or beads on table tops.
For food you could make finger sandwiches, fruit trays or kabobs, chocolate fountain or chocolate cover strawberries,