I'd recommend picking up some books.
Palleta's "The New Marine Aquarium" is a good book for beginners, then move on to the "Conciencious Marine Aquariast" by Bob Fenner.
Keep reading forums, and ask advice. I just bought a 20g system for a $100 from a guy who spent $600 on it - if he'd done more research, it would have done better.
Look for a local marine aquarium club too, you can get good advice and good prices on used gear too. Sometimes free stuff too. I've gotten mantises for free, or for trades from other members of the club I'm in. A bag of chaeto for a mantis is a sweet deal IMO.