be sure to rinse all gravel and decor. aside from gravel, you will need some plants to provide shelter and make fish feel secure.
if doing a tropical aquarium, most fish will require a heater. this will depend on the type of fish you want.
attach a background to tank BEFORE filling with water (much easier than when it is full)
make sure the tank is where you want it, cause it will weigh roughly 10 pounds per gallon (so when full, about 300 pounds). when its where you want, fill it up and put all your decorations in it.
set up your filter. what kind of filter did you buy? if its an undergravel filter, it needs to be the first thing in the tank. most people end of with an external power filter (hangs on the back). if so, put the cartridge in, and set it up.
now cycling- this is one of the most highly debated topics in the fish keeping community. you need time for your tank to aquire some beneficial bacteria, this takes a few weeks. if you are patient, then you will have a much smoother success and your fish will be healthier. rushing the process will kill your fish. there is a sticky on the forum about cycling methods, choose the one that you want and follow the directions. basically, you need something to start the bacterial process. the ammonia levels will rise over a few days-week, then rapidly fall to zero. at the same time the nitrite level will rise, and then fall to zero. then you are almost done.
once you have zero ammonia and nitrate, you can add a FEW fish at a time, a week or two apart.
so of course, you will need some testing kits. (liquid, not test strips)- for ammonia, nitrite, and nitrate. if you don't test, you won't know when the cycle is over, and when to add fish!
now the fun part, is planning the fish you want to keep! take some time, find some you like, learn about them prior to buying them. i would not trust petsmart to help you decide what fish to get, they often sell them irresponsibly. their job is to sell you fish, not to help you succeed! they often sell fish too big or in too small a numbers.
oh how exciting! hope this helps!
also, i website that explains the cycling process better than i can: http://fins.actwin.com/mirror/begin-cycling.html