This is the most important thing you can do so that you know what you are getting into. Figure out the main goal of your tank. Decide what types of fish, other animals, and plants you would like to keep. Be sure these are fish that you feel confident that you can take care of well. Number them in order of how much you want them.
Some fish are very hard to care for properly.
Avoid very large fish, sensitive fish, and rare fish when starting out. Do you want other animals like shrimp, crabs, frogs, snails, invertebrates, etc. Find out everything you can about the species of animals and plants you would like to keep. Search the internet under the common and scientific name. Buy magazines and books with information on those species.