I'd start with nothing less than a 20 gallon, but that's me. If something goes wrong in a small tank, it goes wrong FAST, and you could end up with dead fish pretty quick. 20 gallons is a nice starting point. You can have several fish of different types. What kind of fish do you like the look of? Find out what temperatures they prefer, and if they would get along. Personally, I love tiger endlers, and have many of them. They seem very hardy. Cherry barbs are great too. I also love balloon mollies, although many people consider them to be deformed, but I have two I've had three years now, no health problems, and they're adorable. Female bettas are great too, although a 20 might not be big enough, depending on the bettas' personalities. Sometimes one will be more aggressive, and there's not a lot of room to escape for a more passive betta. I have a sorority of 7 girls in a 55 gallon, and lots of hiding places, so my bettas don't ever "get into it," as they can always get away from each other. Oh, and NEVER put a male betta in with females! I also dearly love Peacock Gudgeons. They are as personable as female bettas. Very sweet, gentle and not at all hyper.
I wouldn't recommend goldfish, as they are so dang messy! You'd be cleaning constantly, plus they need bigger tanks. It really comes down to what fish you prefer, since everyone has different tastes, and also what foods they will accept. (Some types of fish will only eat live foods!) So it really, really pays to do your homework. Don't buy a fish just because you like the looks of it. Too many people wake up in the morning to find a fish or two or three have been murdered during the night by an aggressive kind of fish that should never have been put into the same tank with the peaceful, passive ones. Good luck and let us know what you end up with!