We have made three raised beds so far in our yard, two out of wood and one from wood and left over metal roofing. Below is how to build the wood only beds. You can use any wood although keep in mind rotting and such (Cedar would make a great choice although it is $$) I also gave dimensions based on 8' boards because you can fit them into a car that had a hatch to the trunk although you might need to make a couple trips.
Material List (for one 8' L X 3' W X 23" H Raised Garden Bed
12 - 2" X 6" X 8'
8 - 2" X 4" X 8'
3 - 1" X 6" X 8'
nails or screws
saw
tape measure
a angle square is helpful to make straight cuts
pencil
After you have your materials use the above wood to cut the following pieces:
12 - 19" L 2x4's (verticals for framing all sides)
4 - 29" L 2x4's (horizontal for framing short sides)
4 - 8' L 2x4's (horizontals for framing long sides)
8 - 1/8" less then 39.5" L 2x6's (outside boards for short sides) (double check this measurement before you cut to account for any differences)
8 - 8' L 2X6's (outside boards for long side)
Assembly
1. First build the four sides and then assemble them together.
2. Long sides get four 19" 2x4's screwed between the top and bottom 8" 2x4's (one on each side and two spaced evenly in between.)
3. Short sides get one 19" 2x4 screwed between the top and bottom 29" 2x4 (one on each side)
4. After sides are assembled position upright and screw all fours sides together.
5. We buried one side in the ground to help prevent the box from wracking (shifting side to side) but if you are not sinking in the ground you might want to add two pieces of 2x4 short ways across the bottom of the box evenly spaced to help the box not shift when moving it. Once you get it filled with dirt it will be fine.
6. Level the box (or just eyeball it)
6. Next add the 2x6's to the outside. The 8' 2x6's go on the long side and the "1/8" less then 39.5" boards go on the shorts side. I started at the top of the box and worked down to make sure the 2x6s didn't stick up slightly above the frame and mess me up with the top piece later but this does mean they might stick down a bit low on the bottom of the box. (I'm talking like 1/4" not a big deal imo)
7. Lastly measure, cut, and screw the 1x6's sitting rails to the top of the box. Starting with the short sides so you know how long to make the long sides.
You're done! Woo Hoo!