To create a U-shaped structure (rectangle with a missing side), you will need two pieces of the same size for the legs, and another piece for the span. This piece may be a different size. You will use glue and/or nails or screws to join the pieces together, depending on the type of stresses you expect the structure to be subjected to, and your tools and expertise.
The simplest joint is a butt joint where the end of one piece is attached to the other. This can be assembled by attaching the cross piece to either the legs’ ends or the legs’ sides. Although not generally very strong, it is simple and easy.
The miter joint is the weakest joint and is difficult to cut and secure accurately. It is made up of angled ends on both the legs and cross piece and is all end grain against end grain. It will generally need reinforcement to provide additional strength.
Lap and rabbet joints are strong and fairly easy to cut and join, but they use up more material. A lap joint joins two pieces by removing half of the material where they overlap. This gives a good amount of surface area for gluing as well as shoulders for support. Rabbets provide additional gluing surface, although they are generally not as strong as surface-to-surface joints.
Mortise and tenon joints are more complex, but can be pretty secure. The tenon is inserted into a matching mortise. Usually it is glued in place, but the tenon can be locked in place with screws, nails or pegs that come in from the side.
These are just some basic joints that can be made fairly easily with basic skills, but there are other, more complex joints that can be used to secure three pieces in a rectangular shape. Google how-to make woodworking joints” and you should be able to find these and additional joints you can use.
