I want to keep a Freshwater Aquarium, but it's my first time and I don't have a clue which fish mix?

Mixing different fish is an awesome idea. It is all in what you want to see. Now, whether or not you will be able to get the Tetras to stay away from a catfish, that's hard to say..... Small fish "food", big fish "hungry". I have seen some pretty impressive set-ups in pet stores, where they mixed fish with each other. Go to your pet store, look around, try not to spend spend all your money before you do your research. I personally would go to the pet store, see what is available, get a better insight on how to do this. Learn what you can, be creative ! :) Good Luck ! ! :) :)