We've intended to start a garden for a few years now, but when our girls received pumpkin seeds and a gardening starter kit for their birthdays, we were committed to do something right away!
There are several options for building boxes - I decided to use pressure treated lumber available at the local branch of the home improvement store. I only had a 6' wide space so I made my boxes 2' wide to allow room to access them from each side.
I used 2 x 8 lumber and 4x4 posts to brace the corners. I also cut two of these 4x4 posts longer (about 16" long) on opposite corners and buried them in the ground to stabilize the box and eliminate movement of the box when it was bumped or jarred.
Before placing the soil, I fed the irrigation line into the box and I lined each box with weed barrier fabric. The soil is the "Mel's Mix" made from 1/3 vermiculite, 1/3 peat moss, and 1/3 compost (They recommend 5 kinds of manure but I didn't have that much on hand-- I used dairy farm cow manure donated by a kind neighbor, mixed with chicken manure from another neighbor)