I have seen stores which sell complete square garden packages, but wouldn't it be more fun to build your own? There's an easy step by step guide on how to make yours after the jump.
Step 1: build the frame
Take four wooden boards of the same size and put them together with screws.
Step 2: create the base layer
Use old newspapers to make a bottom layer.
Step 3: fill the box with material
Fill the box with vermiculite, peat moss, and mushroom compost.
Step 4: mix the vermiculite, peat moss, and mushroom compost
Mixing the three different materials gives you the best basis to grow herbs, fruits and vegetables.
Step 5: make a wooden grid
Use six wooden slats to make a grid. You can use nails to fasten then to the box.
Your square foot garden is ready now and you can grow your first plants. You can grow any kind of herbs (basil, mint, oregane), fruits (strawberries, cherries) or vegetables (spinach, lettuce, eggplant).
Mel Bartholomew wrote some very good books about square foot gardening, that I can recommend: