Yeah, I'd echo the "go slow" advice.
Presuming that you intend to keep coral (i.e., your tank's not going to be a "FOWLR" - Fish Only With Live Rock), you will need to learn at least a little bit about water chemistry.
These are the most important parameters, in this order:
Salinity
Temperature
Alkalinity
Calcium
Ammonia/Nitrite/Nitrate (ammonia and nitrite should always be zero, some nitrate in the 5 ppm range is OK and even desirable)
Phosphate
That list is for a tank that's intended for at least some hard corals. If you're only going to keep softies, like zoanthiids, mushrooms, and leathers, calcium is less important.
There's definitely a "sticker shock" aspect to this hobby - keeping a reef tank definitely requires more than just the tank, stand, lights, heater and pumps. So if you're considering budget and what to buy in what order to keep your tank's water parameters stable, I'd suggest:
Good temperature measurement - something like
this or
this
Good, easy to use salinity measurement - I highly recommend the digital instruments from
Milwaukee. They're more expensive than a manual refractometer, but there's absolutely zero guessing what the correct value is, which is helpful for beginners.
Salifert Ammonia
Salifert Alkalinity
Salifert Calcium
Salifert Nitrate
Hanna Checker Phosphate
Again, not all of this needs to be purchased at once. Your most important and immediate acquisitions should be a good temperature measurement device and a salinity checker.
Following this, you may really want to consider an ATO (automatic top off) unit. This isn't quite as critical for fish-only tanks, but will be important when keeping corals.