yep!! lots of advice!
this is the most important thing you will learn about keeping ANY sort of fish-
please read that carefully.
if i could make an immediate suggestion, get the tank set up and fishless cycle (if you ahve read through that site, it explains the nitrogen cycle present in your aquarium). to cycle fishlessly is far farless harmful to your fish than to establish the cycle WITH fish. ammonia/nitrite burns will likely kill most of your fish before the cycle establishes itself.
the easiest way to fishless cycle isto set up the tank, put in a pinch of food daily and wait a month.
if you happen to have already gone out and gotten fish before the cycle has completed itself i woudl highly recommend Bio Spira (and no other product) to add the good bacteria to your water. dont waste your money on other cycling products as they are all dead bacteria, not live.
OR just do a bunch of testing and water changes. keep the ammonia (the first leg of establishing the cycle) under .50ppm and nitrites also under .50ppm. if it steps above that, do a water change.
few things you will want to get regardless of the kind of fish-
freshwater test kit (contains ammonia/nitrite/nitrate/pH/high range pH testers)
'aquarium' salt, not marine
high quality fish food
dechlorinator (i use Prime, love it!!)
next, read up about the fish you want to get. find out their tank size requirements and compatibility with other fish you want. contrary to popular belief, you cant just geta bunch of fish and they will all live in harmony.
also check their water requirements. if you have hard water, stick to fish who like that. if you have soft water (oh so lucky if you do!!) stick to fish who need that. google is really helpful in that area!
some questions for you-
how large is the tank?
interested in keeping live plants?
he know what he wants as a center piece fish? or the general idea of what he wants to see in there??
sorry this is a lot to take in, i tried keeping it short.