Unlike human beings and other mammals, fish do not create their own internal heat. They do not regulate their own body temperature, but depend upon their environment to do that for them. This is why regulating the temperature of a tank is so important.
Most fish have a relatively small range of temperatures they can thrive in. While room temperature may, at times, be the perfect temperature, at other times it may be just a little too cool or slightly to warm for less hardy fish.
Start with learning about the temperature range recommended for your type of fish. This will help you identify what kind of tank heater you should get. Heaters can be set to different levels, to help you get the temperature right.
Some heaters are submersible and others hang on the side of the tank. Perhaps the most efficient solution is a heater filter, which actually heats the water as it passes through the filter, helping to keep the temperature constant throughout the water of the tank.
In order to measure the heat on a daily basis, you can purchase an inexpensive tank thermometer that has a range guide for basic types of fish. For example, tropical fish are the most common fish purchased for in-home aquariums. These fish tolerate only temperate waters of 74-80 degrees Fahrenheit. And a good tank thermometer will indicate that acceptable range by a shade of green.
If the water is getting too hot or too cold, the thermometer will indicate that with first a yellow warning range and then a red range, showing that the fish are in peril if the tank water is not brought back to proper temperature in time.
If you are still unsure about regulating the temperature of your tank, speak to a knowledgeable salesperson in the fish department at the pet store. They should be able to point out to you which heaters would work for your tank and help you find a thermometer that fits your needs.