Assuming that it's cured live rock, you DO NOT want to put it in fresh/tap water.
I like to put mine in a bucket with some egg crate at the bottom, preferably about an inch or two off the bottom if possible. Then I fill the bucket with a higher salinity water (1.030 or higher) and soak the rock. Sometimes it helps to put some fish food in the bucket under the egg crate. The point of this is that the higher salinity won't harm the live rock like fresh water will, but it will cause many critters to leave the live rock seeking lower salinity (more comfortable) water. The food is just an extra way to entice them out. The down side to this is it will attract good and back critters out of the rock, so I check on the bucket every few minutes and pull out any good critters that I may want to keep and throw them in my tank (brittle stars, small bristle worms, etc). You'd be amazed at some of the bad crabs I've caught coming out of the live rock this way.
**Make sure that the rock stays wet from your LFS to your home. If you know you're going to buy rock, it's wise to bring a bucket with water in it and have them place the live rock in it at the store so that you can keep it wet and prevent die off.
