Here is some fish food for thought!!!
Common Newbie Pitfalls
There are lots of bad experiences to help keep you on the right track.
My comments are on understanding how big your fish selections will get, and that as many say here, and I learned only after being on here, that the fish don't just grow to their tank size and stop, they suffer stunting, which is horrible. And you don't want that to happen. Don't just take the pet store's word for it triple check multiple sources. I have been very surprised in learning how big some of those cute little fish can get.
Silver Dollars, most Plecostomus, gold fish only after great consideration and a pond or really big tank and filter system, Catfish careful selection, eel, knife fish Bichter/Dinosaur fish, understanding aggressive, semi-aggressive and peaceful tropical fish should pretty much be kept with their respective type, although there are some exceptions to the rules, and you need to understand how the dynamics work.
For many fish, if another fish will fit into their mouth, they will eat it.
Happy fish keeping.
It is a great responsibility to keep a creature from it's own habit and place it into ours.