It all depends on how much you want to spend and what fish you want?
A 10g could work or you could go with a 20-40g so you have more options.
Basically decide what you can afford, and make a budget. Then figure out the two fish you want (once you get into it you may want a lot more so really it's what you want to spend)
A good thing to do is watch craigslist and/or some other places. There are often people moving and wanting to get rid of their stuff (waiting till just before they move and they will be almost giving it away... if not sold before hand.) or they have just had enough of aquariums and want to get out of the hobby.