I know you asked for a website, but I think that reading the book "The Marine Aquarium Guide - Beginner to Breeder" by Martin Moe is the best start you can get to this hobby. I picked up a copy for $6 on E-Bay a few months before I even bought my first aquarium and I still reference it at least once a week. It lays out everything you need to know in a very straightforward manner.
Other than that book, this is one of the best places Ive found for information. It all seems rather intimidating at first, but once you actually get into it, it really all seems to fall into place.
this site has a good getting started brief
Here is a good site for fish information
I hope this is some help