I am an incompetent gardener as well - that is why I went with a proven design. I considered building them myself too, but was put off by the amount of time I would need to spend cutting plastic.
Before planting anything, and especially to help you with selecting the proper planting mix, please check out the earthbox forum and read some of the stickies they have there.
http://forum.earthbox.com/index.php?board=4.0