How can I fix a leak under my kitchen sink?

Yes you can Find out whats leaking then go to www.diynetwork.com

First you need to dry all the pipes and run water down the drain to determine exactly where the leak is. Most of the time, all you will need to do is remove the connections and reseal them with plumbers tape and/or putty. Sometimes, the connection itself may cracked or stripped requiring replacement. Either one is relatively inexpensive and simple to do.

Blueyes sounds like a plumber. She gave you pretty good advice. I should add that sometimes you can just tighten the connection that is leaking.

put a little water in your sink with stopper in place. Slowly let water out. If you can not locate your leak, using a drop of food coloring like red should show up well on white pvc. If you have a garbage disposal, these vibrations can create a havoc on connections. If water is coming from around the garbage disposal, you should be able to tighten it up to compress the rubber O ring. If not the garbage disposal, Teflon, pipe dope, or putty will do just fine.