After we bought our house, we started looking into ways of removing the copious amounts of gravel covering portions of the yard. When we started figuring how much a truck load of gravel weighs, how much time (and work!) is involved to shovel it out by the wheelbarrow load, and the associated dump fees, we decided to work with the rock. Raised beds were the best option we were able to come up with to reclaim functional gardening space.
We could have never afforded to build these had we not found used cedar 2x6's that the parks service was throwing out and redwood 2x6's salvaged from a deconstructed deck. Those two windfalls yielded enough good rot-resistant wood to work with.
Additionally, I added a soaker-hose irrigation system built into the planters with quick-connect fittings for easy watering.
You will need cedar, redwood, or similar rot-resistant wood. Some people use pressure treated wood, since the new types don't contain arsenic, but some people are still wary. Cedar lasts longer anyhow, and I think it looks good. New, it's really expensive, but look for old decks, playsets, pergolas, etc. that you can haul for free.
Use a good coated, rust resistant screw to assemble. I used 3" coated deck screws. Each layer is assembled with 3" galvanized nails.
6 -- 2"x6"x7'
6 -- 2"x6"x3'
2 -- 2"x6"x7' 10" mitered 45 degrees toward center on each side
2 -- 2"x6"x3' 10 1/2" mitered 45 degrees toward center on each side
8 -- 2"x6"x 16 1/2" for corners
Circular saw or crosscut hand saw
Strong drill or screw gun, preferably corded