Buying a skateboard.

You need the foolowing items for a skateboard:

Decks

Obviously you need one, but the question is, which one is right for you? Well, you have many options. Size should definitely be taken into consideration when making your selection. Before you make that decision, determine what type of riding you do, or plan on doing. This is important. If you skate streets or do more technical tricks, you're going to want to look for a board in the size range of a 7.5" to 7.87". If you mostly skate pools and parks with heavy transitions, then you may want to consider looking for a board in the size range of 7.87" to 8.25". You also have to take into consideration you own size. If you are 5 feet tall, and weigh 80 pounds, you are not going to want an 8" deck. The deck will be hard to do flip tricks on. This scenario can also be reversed. Don't purchase something too small for you. Many manufactures have a specific concave for their deck shapes. Concave is basically the mold or the press that shapes skateboard decks. Predetermined steeepness on the nose and tail, along with the body of the deck can be deep, mellow or anywhere in between. This again reflects personal preference and experience. The graphic also may be an influence in your deck decision. Be sure not to base it all on the graphic because you may become frustrated skating if you get a board that is not sized right for you.
Trucks

You need two to ride. Trucks are fairly easy to choose when it comes to size. There are numerous brands and that choice is yours. Generally the axle width on the truck should be within 1/4" of the deck width. The height of the truck is also a key when selecting what trucks are right for you. The reason you would want to choose a higher truck is based upon your style of riding and selection of wheels (see step 3). Once you start selecting wheels over 56mm, you should consider purchasing high trucks. High trucks allow more clearance from the bottom of your board to the surface of the wheel. "Wheelbite" is something you definitely want to avoid. This is when a wheel comes into contact with the board stopping you dead in your tracks, most likely resulting in a painful digger.
Wheels

Roll with four. Skate Warehouse sells wheels in sets so this is more or less predetermined for you. You now need to decide what brand, graphic, size and durometer is right for you. Your selection of size is the most important. Smaller wheels are generally for more technical and street skating, where as larger wheels will ride better when skating parks and bowls or terrain with heavier transitions. The durometer of the wheel is a numerical value allowing you to determine whether the wheel is softer or harder. Most wheels are in the range of 97a to 99a. Basically the smaller the number, the softer, the larger, the harder. Larger wheels are generally a bit softer. Dual durometer is just what it means. There are two durometers for the wheel. The core is harder, while the outside surface area is softer. There are a couple reasons why. First, the internal area is called the "bearing seat" and by making this area harder, it protects the bearing from crushing or losing its shield. Bearings don't usually crush, but it is an option that some wheel manufacturers pass along to skateboarders. It's also good to point out that softer urethane generally slower than harder urethane. Depending on the durometer, it may make a huge difference or it might not make any difference at all. Cored wheels can also be lighter than a standard urethane wheel depending on the core construction.
Bearings

Bearings are sold in sets of 8, 2 for each wheel. Bearings are what actually allow you to have some movement. Obviously you need all the parts, but without bearings you won't roll. Two Bearings sit inside each wheel, and then each wheel is placed on the truck axle. Tighten the axle nuts and you're ready to go.
Grip tape

One 9" x 33" sheet of self-adhesive grainy grit is all you need. You have choices, but essentially it's all the same. Griptape is applied to the surface of the deck that you will be placing your feet on.
Risers

Risers are sold in pairs, but they are optional. Skate Warehouse includes risers into the cost of your complete, but you have the option to apply them to assembly or not. Risers are anywhere from 1/8" to 1/2" thick and their function is to give you more clearance between the bottom of the board and surface of the wheel. This again helps to prevent "wheelbite".
Hardware

Hardware is sold in a set of 8 bolts and 8 nuts. Hardware allows you to connect your deck to your trucks. Skate Warehouse has a large selection of sizes and brands. The choice is yours.