First welcome to Reef2Reef! You will find lots of good info here! .... now a few questions for you, Are you wanting a FOWLR tank or a Reef tank?
The 90G tank would be great, I actually started with a 75G but downsizing to a 65G due to lack of room.
Lights - A good amount of your money will be spent here. Dont do what I did and buy 4 different fixtures. Get what you want from the start! (Lighting will also depend on what types of corals you want in your tank)
Flow - Get good powerheads!!!
Filtration - Since the tank is reef ready, get yourself a sump or build your own (DIY is much cheaper)
Skimmer - This is another quality item you dont want to skimp on. Get something rated larger than your tank, and make sure it will work for your needs. (You can get by the initial start up process without this)