Wood shelving units can be made in any number of designs. The simplest shelf units have horizontal shelves and vertical sides, like a bookcase. Shelves can be adjustable, removable or fixed. Start with your desired outside dimensions for the unit and then figure how to cut and join the pieces.
Choose your wood. Pine is very easy to work with. Hardwoods like oak, cherry or maple will look better if you are going to stain the shelving unit. Choose a 3/4-inch or 1/2-inch thick board stock. Width of shelves is variable, usually about 6 inches minimum and up to 12 inches or more for bookshelves. Try to find boards the correct width so you don't have to end-join or rip the boards to the right width.
Plan your outside dimensions. Figure out the outside dimensions for your shelving unit. This will dictate the measurements for the top, bottom, sides and interior shelves. For instance, choose a 4 foot by 4 foot square.
Cut the pieces for the outside frame. Make the top and bottom 48 inches (four feet) long. For the sides to be exactly 4 feet high, you have to compensate for the thickness of the top and bottom pieces. Cut the sides 1 1/2 inches shorter than 4 feet (for 3/4" thick boards) so they will fit between the top and bottom. For 1/2" thick boards, cut the sides to 47 inches in length.
Cut the shelves. These will be 4 feet wide, less the thickness of the two sides. For 3/4" boards, cut the shelves to 46 1/2 inches. For 1/2 inch boards, cut the shelves to 47 inches. Decide how many shelves you want and mark the position for the shelves on the inside of the two sides with a ruler and pencil.
Assemble the frame. Use clamps and a square to fit the frame together to make sure it fits. Glue, screw or nail the frame together square.
Add the shelves. Glue, screw or nail the shelves into place along the marks you made. You can make removable shelves by installing horizontal one inch by one inch strips of wood to support the shelves. Add additional strips so you can adjust the shelves later on.
If you are gluing in permanent shelves, it may be easier to glue them at the same time as you assemble the outside frame. Sand all pieces before assembly if you are staining--it is a lot harder to sand the pieces once they are assembled.
Always use appropriate protective clothing and glasses when using carpenter's tools, especially power tools.