The Raised Beds
The typical raised bed is 4 feet wide by 12 feet long, raised about 10 to 12 inches above the ground. It starts with the soil below it being weeded well, then dug and turned. Note that I'm not spending time on details such as lining up the shovel marks, slice depth, etc. You will find these described in many books in the library, along with various techniques of soil care. Many of the techniques involve emptying the entire rectangle of soil down to about 10 inches, mixing the soil with the extra soil and conditioning materials that will go into the raised portion, putting the bracket for the raised be over the hole, an then piling in all the soil. To tell you the truth, this is the best bet. The more consistency of soil and drainage, the better the nutrients are mixed, the better your produce. And since this is your first time doing this, you want the best environment possible so as to eliminate any possibility of failure.
The reason the bed is 4 feet wide is to give you the access from each side you need to tend the plants without walking on the plants. You will walk on this bed. Diagrams for enclosures, by the way, have shown everything from construction bricks (cores filled with dirt, anti-pest plants or basil planted within) to old bookshelves. I personally would stay away from any pressed wood or ply. I’m not sure how the glues would affect the soil, and we want to keep this pure. I have found that discarded wood and some, but not all plastic liners work fine. You can pay as much as $100 for a professional-grade plastic liner. Nice, but that comes out of all those food savings you were looking at. Some folks specify angling the liners in so as to widen the top of the garden. Nice idea, but the construction is tricky. You want a wider garden? Dig a wider garden.
The Religion Starts...Here
I know. You snuck out and started reading about the 14 kinds of compost and their fermentation methods, the pH balancer, the emulsions, the relative quality of ground horn from cattle, elk, and unicorn, the special French tools for forming the soil, the life cycle of the snail, the -- Snap out of it! We're not building a cathedral to floral perfection. We're planting a garden, here. Choose a mulch that will drain well and add nutrients that are the best for your local area. Avoid concoctions, in this first garden, that make vague or undocumented claims, especially the ones that start, "The Chinese (or Hungarians, or Mongolians, or any other distant culture) use this to increase..." Also, avoid anything that increases Chi. Chi is -- never mind. You can increase the planet's Chi after this harvest. First time, let's just do dirt.