When building a raised bed for a vegetable garden, what do I need to do to properly prepare the soil? It is going to be mixed with different types of vegetables and herbs that I will rotate out all year long.
Should I buy the more expensive garden soil? Or use essentially use top soil and add the necessary fertilizers?
I don't know about "properly", but what we have done on three beds (and it seems to work) is:
Start with roughly 50/50 mix of cheap topsoil bags (tend to be very sandy) and the more expensive garden soil/compost. This will settle to perhaps 50–60% of the alleged contents of the bags. So keep topping up over the next few years when the beds are empty.
Although I might occasionally add the cheap topsoil bags to a young bed that requires a lot of topping-up, I usually top up with "Shrub compost" (Lowes sell it in big bags intended for trees and shrubs, but it seems to work very well for peppers and tomatoes); our home-made compost; and "used" potting compost (i.e. from pots which have had plants in them).
I too have no expert advice, but I just built wooden boxes (3'x12') and scooped the topsoil from the paths into the beds. I then laid down sticks, grass, wood chips, etc to fill the lowered walkways. Over the years the walkways composted beautifully.
You can move the raised beds, till the whole thing and put the frames in another orientation if you want.
I added a layer of compost in the fall and spring to the tops of the beds. (before planting and after harvest)
You shouldn't necessarily be worried about the nutrient content as much as the loam of the soil. You don't want a soil that is too sandy, clay-y, or full of too much organic matter. The rule of thumb is to have around 15-20% organic matter and a soil that drains well.
If you live in the desert your best bet is to use top soils/composts from elsewhere. Just create the raised bed using wood or rock, then fill in the area with a mix of sand and soil.
If you live in a place where vegetables have a better chance with the native soil, try just adding compost. You'll still need to transfer soil from one part of your yard to another if you want to use a raised bed (which, admittedly, can help with drainage) but it'll be a bit cheaper to start than all of the bags of soil you'd have to purchase from your local nursery.
From the reading etc I have done on the subject, it is important to get the right balance of minerals as well as organic matter. So you might consult with people from your State's "Master Gardener" Program. They can also usually tell you where you can get your soil tested. Testing your soil every so often is a good idea, so you know what you will need to replenish, or what you may have too much of.
I would suggest talking to a Master Gardener in your State to get their advice.
