Weapon of choice would be telekinesis, with which I would use to infiltrate peoples minds, cause them to hallucinate, imagine scenarios and people, etc.
As for clothes, street clothes would do, a mask isn't necessary because I could manipulate peoples visual perception of me, and I'd absolutely love to throw down with Jason, since he seemed pretty weak when it came to those with thought manipulation powers.
I would take out the virgin first, followed by the cute 'n' sensitive guy the virgin likes (these are usually the two characters who escape and/or defeat the killer). And if anybody's father is the sheriff, I'd probably be better off finding another group of horny teens to slaughter.
Also, I wouldn't disembowel them and leave them hanging to alert the other teens; I would dispatch them all quietly within an hour, bury the bodies, then go home and watch TV.
Hey, somebody should come up with a "how to be a slasher movie villain" list, like Peter Anspach did with the Evil Overlord how-to.