My friend is selling Guppies and...?

Get three males. The males are prettier than the females. If you're only getting three you don't need a huge tank. You'd be ok with a 5 or 10 gallon. You need a filter. That's how you keep the tank clean and aerated. You can get a kit with the tank, gravel, and filter from any pet store or even Walmart. Honestly though, my friend's guppies were always having babies but the adults eat them so most never survived.