Alright, tsifuentes. Wow! You really got whisked knee-deep into this fine little hobby, didn't you?

:
Here's my take: your tank is too small to house 5 goldfish. They get HUGE very quickly. Some might even argue that your tank is too small for just ONE goldfish! My recommendation would be, if you can bear it, to take the goldfish back to the store and start up a really nice tropical community.
Tropical communities stay at around 76-78 degrees, typically. What you'll want to do is buy a heater, though, to ensure that the tank's temperature stays constant, which might be even more important than the actual temperature itself. Without a heater, the water temperature will fluctuate with the temperature of the room where the tank sits. Constantly changing temperature can be very harmful to a tank's inhabitants.
Now, on to the ammonia. Ammonia is indeed quite harmful. You can reduce the level of ammonia by doing a large water change, meaning changing out 50-70% of the water in the tank with new water. BUT! But, when you do this water change, be sure to have your bottle of Prime handy (congratulations, you bought what most aquarists hold to be the best water conditioner!

). Prime really only takes the chlorine and chloramine out of the water. The only thing that can remove Ammonia, Nitrites and Nitrates from your water is to perform water changes. Water changes, as you'll find out, are an integral part of keeping your tank healthy along with gravel vacuuming and eradicating disease before it has a chance to establish itself in your tank.
As for what to do right now, goldfish prefer temperatures around 70 degrees. They are coldwater fish and do not do well in a tropical environment. I hate to have to tell you, but your tank is not big enough to comfortably house goldfish. If you keep them, they will be stunted and die live a short, painful, miserable life. I might recommend liveaquaria.com as a place to browse the types of tropical fish that are more suited to your tank setup. There are many gorgeous small species out there that you could successfully keep and enjoy in your 20 gallon tank!
I hope that I've helped you with your situation. If you still have questions, please feel free to private message me, or post your own thread in the newbie forum instead of staying in this particular thread.

People around here love to help (usually) and would be more than willing to help you.
Good luck!