The first thing that must be determined is the type of enclosure that you want to have. There are four types of tanks that you can have:
Saltwater: this is exactly as it sounds, a tank filled with saltwater. This means that you are going to be housing things that come from the ocean. Within this group of aquariums you could house fish, live rock, corals and invertebrates. You can mix and match any of these things into the same tank you just need to understand the relationship between the various groups, which will come in a later post. These types of tanks are very popular as they usually have a large assortment of colourful fishand corals. They are more expensive to maintain and fill, as well as being harder to maintain as they require specific salinity and chemistry to maintain. Because of this they are more frequent with companies and movies and the affluent. The ocean is vast and containing it in a small space is a large endeavour.
Freshwater: this is a tank filled with, you guessed it fresh water. This means more species that come from ponds and lakes. These tanks usually have fish, plants, invertebrates and amphibians. You can mix and match, again as long as you understand the relationships involved. These tanks are the most common tank setups due to their low cost and maintenance. They are relatively easier to setup and the species are generally low in cost. This is the perfect place to start you aquatic adventures for any age. Freshwater tanks have a very large assortment of fish and can be quite colourful as long as you look for the proper fish, making them as enjoyable as the marine counterpart.
Tropical: these are tanks that can be either freshwater or saltwater, what is important here is temperature. Tropical tanks usually try to maintain a temperature between 24 to 27 degrees Celsius (75 to 80 Fahrenheit). This is done with the addition of a heater. The best place for the heater is next to the outtake of your filter, this way it circulates the warm water around the tank creating a more even temperature throughout the tank. If you place the heater next to the intact you waste a lot of energy and heat as the warm water travels through the filter, and if you put it nowhere near the filter you run the risk of creating a hot spot on one side of the tank.
Temperate: these tanks are usually tanks that maintain a temperature below the 24 degree mark. These are also some of the less expensive fish, such as goldfish and some koi fish. They do not require a heater in the tank, but the aquarium is more sensitive to temperature fluctuations in the room. Which means if the tank is kept in a basement where the heat is not always on the temperature in the tank will eventually meet the room temperature, thus fish will be going through more temperature changes then ones with heaters in the tank.
Whatever your pleasure, remember that each option has its own pros and cons. It is always best to start with the most popular so the prices are better and support is more, once mastered one area then move to another. Most aquarium parts can be used in any combination of the above.