First thing is: locate the leak. A pinhole leak usually shoots water in a stream, so look around the pipe for a stream of water, but be careful, as the water will likely be very hot.
Once you've found the leak, shut off the water to the house at the water main. Then, open a faucet somewhere in your house to relieve the pressure from the system (this is an essential step; if you don't do this, later on when you're putting on the epoxy to plug the leak, the pressure in the system will cause the epoxy to bubble and you'll be back at square one).
To plug the leak, I use epoxy; this is a glue-like substance that is about as strong as PVC when it cures. You can find many different kinds at Home Depot, I've been using the Loctite brand (for some reason I've been using the marine epoxy, which is supposed to be able to cure under water; I don't really think it's necessary though. Any old epoxy should be fine).
Take some sandpaper and buff the location of the leak; this will create a better surface for the epoxy to adhere to. Then, mix your epoxy and spread it over the leak. You'll need to wait several hours (even though it says it sets in 50 minutes, you'll want to wait at least 3-4 hours, possibly more; if you turn the water back on too soon, and the epoxy is not hard enough, the pressure will punch a hole right through and you'll be back at square one, again).
And that's it, no need to call a plumber! Make sure to pick up any standing water with a wet/dry vacuum, and possibly bring in a dehumidifier to suck up any excess moisture (to prevent the growth of mold).
