Welcome to a fantastic site to get information. And don't worry about doing something wrong - all of us here had to learn about forums at some point in time. I agree with everyone else, check out the material on cycling a tank before you start adding more fish. The question you asked about how many fish can be added to a tank should be placed in the freshwater forums. I say this because not every member checks the Welcome Forum and you would be short changing yourself for the full benefits of knowledge this site has to offer. Make sure you tell us aside from the size of the tank, what filtration is in place, pumps, water parameters, etc.
But for a quick answer, there use to be a rule - 1 inch of fish for every gallon of water present in the tank (the 1 inch should be estimated on the fishes expected adult mature size).