Sounds like a little reading on cycling the tank might come in handy. Here's a good article...
Before you start stocking the tank with critters, you need to make sure there's a population of bacteria living in the tank. That bacteria is really important because it's going to do the job of converting the ammonia (from fish poop, fish breathing, leftover food rotting away, etc) into something harmless to the fish/coral/crabs/etc. Ammonia is toxic to live stuff, but the end result (nitrates) is NOT toxic. And that's where the water changes come in... to get rid of the nitrates.
Read over that article, and also the ones in roka64's signature block, and hit us with more questions. We've all been down this road and have asked "dumb" questions ourselves.