First thing's first; Welcome to AA!
Now on to your tank...
1) The tank you see there is actually not really what you want. I had (still do, just modified and made better) a tank like that. It's bad for a few reasons. One, the gas exchange is poor leading to pH troubles, and Two, the lighting is almost always terrible. Mine had two fluros over it. I bought a great T5HO (HO
= High Output) light unit and seen the difference. So try for a plain old glass tank, which cuts out those problems.
2) Are you aware of the costs? Depending on the quality of the things you buy your tank will probably cost you at least $1000 once you've set it up and got some fish and corals. Lights, salt, live rock, sand, heaters, powerheads and livestock. It adds up quick. Searching places like eBay and Craigslist can get some bargins but somethings will never be cheap.
3) In terms of step by step, first you need the tank. Unless you already have it. If not why not go for a bit more and buy a 30G or even 40G. It makes it easier because the larger water quantity makes the levels more stable. Once you have the tank you need everything else. We can't help you with everything else though until you have the tank because we don't knwo what's good for your tank!
So good luck. As well as reading the links Mike (melosu) posted above why not checkout some other build threads of small tanks and learn from similar stories.