What is a raised bed? A raised bed is simply a box on the ground that holds soil for plants to grow in. It can be used where the ground is too difficult to work with for growing, where space is at a premium (with the right mix you can grow plants in a raised bed in a much greater density than in the ground), and where weeds can be a problem (because it can keep them out).
Size. A raised bed can be as big or as small as you want, although you'll need access to the center of the bed for planting, weeding, and harvesting. A popular bed size is 4 feet x 4 feet. Other common sizes are 4x8 and 3x8.
Depth. This bed is about ten inches deep, but you can make a raised bed as shallow or deep as needed. Consider the root system of the plants you're growing and the method of gardening you're using. Some methods, such as square foot gardening, require less depth than others.
Materials. Choose a rot-resistant material for the frame of the raised bed. Also, if growing food for eating, avoid any treated lumber. We used cedar logs since we had some from our own land. Definitely use the resources at hand if at all possible. Stone is another possibility. Cedar and redwood are popular types of wood for raised beds, as they are naturally rot-resistant.
Once you have your dimensions and your materials, lay cardboard on the ground on the site of the raised bed. If weeds are an issue, consider using 1/8" hardware cloth (woven wire mesh) as the bottom of the bed as well. That's what I did in the picture above, since I'm trying to grow strawberries in an area full of quackgrass and other heavy weeds that would take over a strawberry bed.
Then, cut and fasten the four sides together in place on the site of the bed. The specifics of this will depend on the material you have chosen. For wood, use a 2x2 piece on the interior corner and screw each side into that.