Once you've decided more or less what you want to grow, it's only a matter of siting and designing your bed. After that, comes all the labor of actually growing stuff. In this post, I'll briefly examine some options folks have for designing and building their own planting beds.
To make it somewhat reductive, you've got open beds and raised beds. While there's nothing wrong with digging up some lawn, plunking down a few bags of fertilizer and getting to work, I'm going to recommend folks use raised beds or at least line their ground-level beds. By using some kind of wooden or stone liner, or just planting in a raised bed, you don't need to worry about grasses and roots choking out your vegetables' as much. Also, some plants are going to much better in their own enclosures, like the spud box.
So when designing the bed is site. What you're growing (and what you may grow in the future) will determine how much sun your plot is going to need. For this, just check the seed packets and see what the most common required sunlight values are. For the especially fastidious, you can also set up a camera on a tripod to take a photo of your proposed site every hour to get a sense of how much shade is cast on the spot throughout a sunny day.
Then, material: One of the most common mistakes I've seen folks make is line their vegetable gardens with treated wood, to prevent rot or termites. DON'T do this. The treatment that is sprayed on those planks will get into your soils and, qua, your vegetables. I exclusively use stone or brick, but if you can get out to a local lumber-mill and request some un-treated wood, that will work as well [though, you may have to replace the planks every few years].
So once you've got your site and a lining material you're happy with, dig out the plot by hand or rent some landscaping tools to tear up your plot. Then, set the liner in about one to two inches into the soil and pack in earth on both sides to get them to sit still. Fill your bed with your fertilizer of choice and you're good to go.
For those looking for a longer step-by-step discussion of what I've discussed here, check out Howard Garret's book or website for detail.
In the next post, I'll address good pairing plants and some ways to deal with pests once you've gotten started.
Works Cited
Mabel, Joe. "Picardo Farm, Wedgwood neighborhood, Seattle, Washington: A community allotment garden with raised beds for the physically disabled." Wikipedia.org, Wikimedia., 28 Oct. 2006. Digital photograph.