How do I find a plumbing leak inside my wall?

I hear a dripping noise that sounds like it is behind my bathroom sink any time I use that sink or flush the toilet on the same wall. Also my water bills have been high lately so that seems to reinforce the idea of a leak. I don't have the money to hire a repair done if I can possibly do it myself even without relevant experience. I assume I need to cut a hole in the wall to see in through, but how do I see beyond the hole so the hole doesn't have to be large? How do I avoid accidentally cutting the pipes I can't see behind the drywall as I'm cutting it? Then what?