I would start by reading all the articles in the articles section before you do anything. Then when you think you've read enough, read more and then read more and then finally read some more. I wish I would have read and learned more before diving head first into this hobby, I've wasted hundrends of dollars because I didn't know what I was doing.
A few questions for you first?
what size tank do you plan on going with
what do you plan on keeping, fish, inverts, corals
what is your budget, that will help determine the above questions
I knew this hobby was expensive when I started, however I had no idea..... whatever you think you will spend multiply that by 10.
Just for example, my tank which is 110G which I'm starting to gradually add corals, I've spent roughly $600 on lighting alone so far, and that nothing compared to some others on here.
Some things you'll need to consider in your costs
if you plan on anything more than fish only
sand recommended over cc
for your substrate
various test kits, amonia/nitrite/ph
for sure plus a few others later on if you decide to go reef
possibly a skimmer
various plumbing supplies
Quarantine tank if you are smart and don't want to learn the hard way like I did
can't think right now, I'm so tired from finals week but I'm sure others will chime in. Its all the little things that add up that will kill you when you're not expecting it.
Good luck with whatever you decide to do, I'm not trying to scare you, just letting you know what's ahead. This is a very rewarding hobby once you've had a few upsetting frustrating times trying to figure out why things are working the way they are "supposed" to. I've been ready to tear by tank down a few times but I've learned so much from being on here, now I love this hobby and my 5 tanks. I spend countless hours just watching them. Wouldn't trade them for anytihing