I agree with Woody
Research your fish!! Know what fish you want before putting them in the tank! Also don't buy a fish that is too big for your tank, even if they are babies! Most people make the mistake of buying a small little baby fish and throwing it in a tank too small for it. Thinking they will upgrade soon. They never upgrade and a Blue Tang ends up swimming in a 29 gallon tank for longer than it needs too. Remember when you buy fish, you buy them as babies and not their full adult size! Always buy a fish with adult size in mind, and never buy a fish that will soon outgrow it's tank thinking you will upgrade soon. Buy fish that can live their full lives in the tank you have!!
Throwing it out there because of importance! DON'T be fooled by bottle bacteria! The stuff does not work like they say it does. It can be ok to add when your cycling your tank, and as a booster for water changes, but that's about it. You will need to go though the cycle without fish for a while. Mine took 3 weeks to cycle!
Next add fish slowly. One or Two at a time so you don't overload the bio-load in your tank. I added two clown fish to my tank as the fist fish, because they were a pair. I waited a whole week and will be adding a fish tonight. Slow wins this race!
Buy the good equipment not the cheapest you can find. You will save money in the long run!! Plus your fish will be a lot happier.
Depending on your tank size add that much live rock. I have a 29 gallon tank and I added 50 pounds of live rock. I spent a little more doing it, but my Bactria colonies are strong and my filtration is awesome. If you have a 29 gallon tank, by at least 29 pounds of live rock, and get more base rock. You can seed the base rock with the live rock, so it can all become live rock in about 6 months.
A nice sand bed also helps. I know some people go bare bottom, but to me that looks tacky and I don't see any benefits (Easier cleaning, I guess). A nice deep sand bed can also help with filtration. I got 60 pounds in my tank. I might have went a little overboard.
I might have went overboard in the Live sand and Live rock department, but it's my first saltwater tank and I don't want it to fail. I did about 5 months of research ahead of time while saving up for the parts and tank. I am still working on it, as all the fish are not in yet.
Last but not least, and I know this has been said but want to make sure you know how important it is. Cycle your tank before adding any live animals in it. You can do this buy setting up your tank as if you had live stock, with everything running. Get a clean pair of panty hoes (New ones), and go to your local supermarket. Get some uncooked raw shrimp and put them in the hoes. Set it in your tank and let nature take it's course. You will notice a white slime and it will get nasty! The panty hoes are so you can take it out once the tank has cycled without the shrimp causing a mess in your tank. I didn't do it with panty hose and I was picking out shrimp guts for a week.