How do you take care of aquarium fish?

That's an incredibly broad question. I can give you some basics, but there is not substitute for reading a book.

The first thing is...obviously you need a tank. I recommend nothing less than 55 gallons to start out.

Place the tank away any place where it will get direct sunlight or be in the path of drafts.

You need a filter. The filter for your aquarium keeps your fish alive by breaking down their waste. Always buy the best filter you can afford, it will pay for itself many times over in the coming years. I recommend AquaClear brand.

You'll need a heater to regulate the water temperature. You will also need lighting and an aquarium hood. You can usually buy a package deal with everything you're going to need at the pet store.

You will also need a water test kit to test for ammonia, nitrite, pH, and Nitrate.

After setting up the tank you'll need to add water. You must use a water conditioner product that removes chlorine and chloramine from the water, as this is deadly to fish.

Once adding the water let the tank run for at least 24 hours to let the temperature stabilize. Then you can add your first few fish. No more than 3 to start. Completey ignore any kind of fish the person's at the store suggest. For some good starter fish, take a look at a book on keeping aquariums.

Your fish will provide the intial waste to begin cycling your tank. Again, best desbribed by reading a book.

You'll need to feed them a small amount 1 time a day, change at least 50% of the water weekly. Scrub algae, prune your live plants etc.

I agree with most of this. The main part I agree with is reading up on the subject before you start. I started a 75-gallon freshwater aquarium and it took me two weeks to get it right before I added my first three fish. Took another five weeks to add the rest of my fish three per week.