The humble hobby of tropical fish keeping is addicting and a hobby easily kept for life. Tropical fish are energetic and loving animals and many species of fish like Mollies and Platys will actually swim up to the tank towards you to greet you.
Beginning, care for tropical fish is certainly harder than it looks. My fish are very needy and take advantage of my constant care for them. Feeding for tropical fish is generally simple, though, a heaping pinch of tropical flakes a couple times a day usually does the trick. When the fish get older, however, they need a varied diet - proteins are very important for them. Although the flakes provide plenty of protein for them, they need a little more than meets the eye with flakes. One snack that my fish love is lettuce - boiled for ten minutes in water (for easy chewing), then strained, and put in an ice cube tray to freeze. Once these tasty treats for our fishy friends are done, put some lettuce in and the fish will chew on it for a long while and the lettuce won't foul up the tank. Another important thing - though fasting for 24 hours once a week is helpful for fish, mine started eating each other because of the absence of their food.
Next, cleaning the tank is very important to keep the fish alive. A 10% water change is recommended for established tanks (tanks that have gone completely through the nitrogen cycle), meaning that if you're tank is ten gallons, take one gallon out and replace it with another gallon of clean water. Depending on how messy your fish are, you're going to need to vacuum the gravel to get the waste out - the fish poo, leftover uneaten food, etc. One tool that I think is essential for aquariums is something called a MagFloat (buy it at your pet store), an algae scraper that attaches to your tank with a strong magnet on the OUTSIDE of the tank. You simply move the magnet around and it moves your algae scraper and cleans your tank. These nifty gadgets run around $15 dollars for the large ones and $8 for the smaller ones. It's well worth the money because the other cheaper magnetic algae scrapers don't float, causing you to stick your arm in and reach down to get to your algae scraper, and depending on how big your tank is, this is a hassle.
Continuing on, your fish need chemical balance in the tank to keep them healthy. Different fish tolerate different levels of ammonia and high pH levels, etc. If your pH (how acidic your water is) is high, your alkalinity is high too. It's important to buy a water testing kit at the pet store to make sure that your chemicals are in the right place.
Additionally, your fish may become ill. This usually happens to pretty much every aquarium owner of any expertise level. A common disease that fish get is ich - white spots on the fish that cause it to itch. Most fish with ich rub themselves against the gravel to scratch their body and this can be harmful to the fish. Fortunately, ich can be cured easily by putting in medication that can be purchased at the pet store. An example of a horrible fish disease is dropsy - also known as "pinecone syndrome". Fish with dropsy swell to the shape of a pinecone (big and round) and their scales poke out, looking like, well, the sides of a pinecone. Unfortunately, by the time you know your fish has dropsy, your fish usually cannot be saved. The best way to help your fish out is to euthanize it, to put it out of its misery, which causes the owner lots of heartbreak.
Summing up, the hobby of tropical fish keeping is fun! Tropical fish are easy to care for once you know what you're doing. I would highly encourage you to try tropical fish keeping out because it is an activity that builds responsibility in kids and adults alike, it's an enjoyable thing to do, and it gives you a heart for fish.