When you go to pick out your first fish tank, you are barraged with an onslaught of advertising. Here in this aisle is a 55 gallon acrylic combo; there is a 70 gallon all glass standalone, and over here is a 70 gallon combo! Truly, for a beginning aquarist, all this information might be confusing. Here, I will try to explain about the different types of tanks and packages.
Glass tanks are usually the best for the majority of uses. They have the advantages of being cheap, rather strong, and scratch resistant. The tanks are usually made from five panels of glass(four sides and the bottom) glued together by silicone and reinforced by a plastic frame. The frame serves little structural function, but it does provide a little bit of bracing.
Acrylic tanks are far more expensive, but they have some advantages. They are made of solid, clear plastic without any glue. They can be made in many different shapes. This can make them fit better into the look of your home, but for the majority of uses, the standard rectanglular shape is the best. Acrylic does scratch easily, but light scratches are removable(removing the scratches is a lot of work, though). It is impossible to replace a single panel in an acrylic tank, as it is made from a solid piece of plastic, so if you get a deep scratch on the front of your tank, there is little you can do but buy a new tank. Acrylic tanks hold heat a little better, saving you some money on the energy bills. But to most people, their benefits is the lack of seams, which makes them look a little better than glass tanks.
When picking a tank, remember -- surface area is the key. That does not mean you should get a 2 inch tall tank; rather, it means that you should avoid tall,short and narrow tanks. Surface area is important for several reasons. First, the majority of gas exchange occurs on the surface, and the larger the surface area, the quicker oxygen can be replentished. Second, tanks with larger surface area are longer and wider, thus giving fish more room to swim. And if you are using an undergravel filter, a larger surface area means a larger bottom, which means more biological filtration.
Size is important, and a bigger tank is better. However, do not get a giant tanks if you are a beginning aquarist and are not sure about whether you want to keep fish. A good beginner's tank is a 20 gallon -- it is big enough to support a reasonable community of fish, and small enough to be managable for a novice. When you get the tank, do not go for the cheapest one you find, as not all tanks are created equal. Remember, 20 gallons is a lot of water, more so if it is on your floor. So be wary and get a quality tank.