Well, the rule of size is one inch of fish (total full grown size) per gallon of water. So you could have 20" of fish.

Do spend the money and get yourself a heater and a good 30-40 capacity filter (as they said, overfiltering saves you trouble). Heck, if I could I would send you my extra heater and air pump. Where do you live?

As for good fish species, in a tank that "small" the best choices would be the smaller community fish such as tetras, platys, guppies(single sex!), and swordtails. Also, having at least one loach or pleco will help cut down on cleaning duties as well. Make sure that whatever you choose, you have the tank setup and running for at least three days before adding fish to cycle the water.
Visit your local stores to get a look at the different fish and see what catches your eye. Then, make note of the species names and do some research online about them. Fish are all about available space and what the owner wants to see.