There is a bit of a science behind all this, and there are so many decisions to make that will affect your long term goal.
I would very strongly suggest you speak to a reputable dealer and get advice straight from the horses mouth.
For starters, your tank needs to build up bacteria and whatnot, and to get this right is critical to the fish's survival, you have to clean it a certain manner to keep some of that bacteria alive, etc, etc.
Another thing that I learned the hard way, you should decide now what type of fish you want as well, apart from salt water fish (don't know to much about it), fresh water fish fall into two categories, "friendly" and "hostile".
We made the mistake years ago to go for the "friendlier" fish since they are much easier to raise and you have a much lower mortality rate. The down side is that these fish tend not to be so pretty and colourful as the more aggressive fish.
The aggressive fish are much prettier, but they tend to fight a lot with each other for dominance of the tank and you end up replacing fish a couple of times (because the other fish were killed or crippled) until your tank settles and everyone signs a peace treaty.
We mixed the two in the beginning and all that was left after two weeks (of 12 fish) was the algae eater and one of the more aggressive fish. For example, the fish that caused us the trouble was a Dwarf Gourami. It chased the more docile fish around the aquarium and at first we thought this lively and entertaining. But one after another the fish died and after chatting to the local expert he informed us that the Gourami was chasing the other fish to exhaustion and that is why they die. I had to put an end to it, most beautiful fish, but it killed of my entire tank, twice.
All in all we went through around +/- 30 fish until we had a stable aquarium.
After this we switched over to the more friendlier fish and our tank has been stable going 5 years now. I'm a bit disappointed we did not stick it out with the more aggressive more colourful fish, but it will cost too much to switch over now and I do not want to kill off the fish we have now.
Yes I know, they can coexist, but that takes trial and error, a lot of luck and know-how to get the balance right.
This is very broad and like I said, go talk to a dealer who knows his stuff.