You need to decide if you want to do Fish Only With Live Rock (FOWLR) or a reef tank. If you want a reef tank, then you need to figure out if you want to do softies, LPS, or SPS. This will all affect how you want to run your tank and what equipment you need. For starters, I'd do FOWLR.
My advice for price: Scour craigslist until you find a good deal. Usually you can get a tank and a stand for a few hundred (or less).
Other than that you will NEED to get:
Lighting (appropriate for your tank life)
Heater (5w per gallon I believe is what you need.)
Filter (skimmer, HOB, there are a few types, I'm sure the others can give you more advice)
Powerheads
Rock! (generally 1.25 - 2 lbs per gallon. I prefer buying the rock dead and then seeding it yourself, but that takes a while (it is cheaper))
Things you will WANT to get (most of these make tank life easier and/or are easier to put in NOW than if your tank is set up and running)
Sump (and a refugium)
Any reactors you want (calcium, algae, ect.)
Uhh that's all I can think of, I'm sure the other people here will have more to add.