Welcome to fishlore! If there is anything we can do to help, just ask. First thing to do, before you buy your tank or fish, read all the posts in the Fish Tank Beginners Guides the link is at the bottom of each page! Much easier to research before, then like neville said get the biggest tank, and filters etc that you can afford. If it is a 5 gal, well, just never overcrowd, and you will still love it.
This time in the hobby, (after taking 30 years off), I started with a 55 gal. , and now am going up from there. Have FUN!!
Welcome to FishLore! It's great to have you with us! Please click on the link in my signature "Fishlore Articles for Beginners". It will help you a lot in getting started. Make sure you understand the cycling process before cycling your tank and getting any fish. We will be right here helping you get the tank going!