The best thing for you to do is to buy a book. There's a great book out there that tells you about every kind of fish, how hard they are to care for, what their max size is, and all other info you need about them. The name of the book is Marine Fishes: 500+ Essential-To-Know Aquarium Species (The Pocketexpert Guide Series for Aquarists and Underwater Naturalists, 1) by Scott W. Michael. It's amazing!
The general rule of thumb for how many fish is measured in inches. In the beginning, you want to have about 1" for every 4 gallons (in your case, about 12 inches of fish). After your tank becomes more and more established, you can move up to 1" per every 2 gallons (this would be 21" of fish). This is the absolute maximum amount of fish you want in your tank, otherwise it will become overcrowded. This can cause your filtration system to overload.
Once you figure out what kind of fish you want, post them, and you'll get a lot of information about each one, and if they're all compatible.