my first advice: RUN! run FAST and FAR! (these are the words of a person who has 14 SW tanx).first thing to know is what critters pique your interest? do you like reefs? fish? a mix?
personally, to give it a first shot, you might want to go with a FOWLR (Fish Only with Live Rock).
the reason i suggest this is that it will let you get used to the mechanics of keeping SW, you can get by with NO (Normal Output) fluorescent lighting (which is cheap), and generally speaking, fish have much more margin for error than coral.
i'd get as large a tank as possible as you'll always want a larger one, but more importantly, a larger volume of water is more stable than a smaller one, so changes happen more slowly and let you get a handle on them before they become "critical" (my first SW tank was a 55 gal).
with many folks leaving the hobby due to financial issues, there are some great deals out there right now, some for pennies on the dollar, others for FREE. for instance, i know a guy who is giving away a 70 gal setup...i have another friend who just got a 125 gal and a bunch of LR for free. the deals are out there...
think about what type of critters tickle your fancy and then go from there. build your setup to suit them and there will be fewer "oh cr@p" moments.