Natalie,
Take Lou's advice. He has pretty much covered it all. But I'm going to explain a little more in detail WHY this isn't the best thing for your fish, so that you may truly understand the current situation.
A 5 gallon tank is very small indeed. Hardly any fish can live in such a small volume of space. 5 gallons is like a tiny cardboard box for us. Imagine spending the rest of your life there, but with other people too. Eventually, you'll be fed, and all of you will start to defecate. Now after a while, all of that waste will start to cause infections and eventually death, unless you clean it very, very often.
The same principle applies to fish tanks, the smaller the water, the harder it is to keep it clean. Especially when you've got too many fish in the tank. I know, every pet store has all of their aquariums crowded, but that isn't the best life for a fish. Just because it can fit in a 5G, doesn't mean that it is happy, again, go back to the box analogy.
Because poop works differently in water than it does for us. You'll have to read about the nitrogen cycle. I'm not going to do it here because there are millions (literally!) of people who have explained it before. All you need to do is go on google and put: Nitrogen Cycle help? And you'll find many resources who will explain the whole process to you, but Lou has already given you a simple summary.
I recommend either:
1)Getting a 40G tank and a heater (after having done research)
2) Giving the fish back. If you're not planning on changing anything, your fish will be very very prone to illness and death.
I suggest returning the fish, then you need to read about each individual species on the internet, what food do they eat, what substrate do they like? Are they aggressive? Are they compatible with X species? How large do they grow?
Keeping fish isn't simply putting a fish in a bowl and feeding it every so often, it's a lot more complex, which makes it a rewarding hobby. Fish are animals nonetheless, I wouldn't keep a litter of puppies in a cardboard box all of their lives, in the same way I wouldn't put so many fish in a cramped aquarium.
Best of luck, keep us updated