Welcome Dragon,
You definately have some reading to do like Newreefer said. Bigger is better, allows for more margin of error and besides you will want to upgrade very quickly (speaking from experience here). I started with a 12 gal, oh I wish I would have started bigger
You need to decide what you want in your tank, corals, fish etc. general rule for fish is 5" of ADULT fish per 1 gallon of water.
Use RO/DI water or bottled DISTILLED. Tap water will cause you nothing but problems, I will tell you that Upfront....
Here is some reading for you.... Read it all, then read it again. post your questions as you get them, and we will be happy to help
LOOK! Frequently Asked Questions~
and here is an acronym list, you'll need it, print it out
Acronyms / Abbreviations
Again Welcome to the addiction