if this is your 1st time i would recommend with a fresh water/tropical setup ..
its easy to take care of ..
there are many selections of fishes to you can start off with ..
koi fish - gold fish is your most basic fish you can start with ..
tropical i would say some tetras and neons and cat fish ..
if you wanna do salt i suggest starting with a 10 gallon and get a couple of fishes ..
but its all on what you wanna do ..
oscars are pretty good too and its pretty cool to watch them eat ..
i had some oscars - arrowanna - cat fish - alligator gars in a 250 gallon set up ..
then i had to upgrade to a custom built tank which held up to 500 gallons ..
but best thing is start off little then work your way up to bigger stuff ..