So aspirin is salicylic acid which is the same ingredient that's found in many over-the-counter pimple preparations.
If you want to use salicylic acid on a pimple, this is what you need to do. Crush an aspirin and mix it with about a tablespoon of water until you get a nice thick paste. Then you can apply it to the pimple and leave it there for a few hours or even overnight.
The key thing if you're going to do this is make sure that that salicylic acid does not get on normal skin because it's very irritating. So make sure you dot it right on the pimple and nowhere else.
I'll be honest with you. If you have a really big pimple, this is not my favorite way of treating it because I see too many people who really irritate their skin by doing this. It's more of a last resort treatment.
If possible, the best thing to do is see your dermatologist and have him or her inject some cortisone into the pimple. You'll be amazed how much it will go down in 24 hours.