When starting a fish tank, the hardest thing to decide may surprisingly be the type of fish to raise. Even though the funniest thing about owning a fish tank may be choosing the types of fish. Choosing fish can be a difficult and fun task, but if it is done incorrectly can cost a lot of money and time. There are many combinations of fish to chose from, but narrowing down these choices is fairly easy. To narrow down the choices, you need to decide the types of fish you would like to raise.
A common choice that people make is to raise fish for offspring. When fish have offspring, it can be a very fun part of raising fish. I would suggest a peaceful tank so that the fry will survive. Mollies and Guppies are good livebearers to start with and easily breed. They have many offspring, so the chances of some of the fry surviving are high. Adding some bottom feeders is also nice, I would suggest some Cory Cats and Plecostomus.
Community tanks are, in my opinion, the funniest type of tank to own. With a community tank, you can have many types and varieties of fish corresponding peacefully. For a good community tank, there are many possibilities. I would suggest colorful fish that actively swim around the tank. Barbs, Angel Fish, Kribensis are my favorites. For bottom feeders, Loaches, Cory Cats and Plecostomus are my favorite.
If you just like a certain type of fish, or would like interesting fish to own I would suggest Cichlids. African Cichlids are very fun fish to rise, but can be very difficult. Finding the right type of Cichlids can be difficult and the wrong kind can cause fighting. Only get the same species of Cichlids in a tank, and you will be fine. Cichlids are egg layers and very protective of their young.
Many fish can be combined with others, but there are some types that just are a pain to rise, no matter the combination of other fish. Bala Sharks are very cool fish, but get ich very easily and can infect a tank, I would suggest not buying them. Chinese Algae Eaters are very good at eating algae, but aggressive and suck the slime off the other fish, I would never buy one. If you need a good algae eating fish I would highly suggest a Bristlenose Plecostomus.
If you are new to fish raising, I would suggest using a few of these combinations to start with, you can add more fish and experiment as your tank grows. I would not start out with aggressive fish, these fish can be very difficult to own even if you are experienced. Trying new combinations of fish is good, but after you add a new fish, watch the fish to for a while to make sure they fit in.
Roshan Goodman gives help on fish and on fish tank advice.