Great idea if you've never tasted veg like carrots or peas cooked within minutes of picking, boy are you in for a pleasent surprise!
As for preparing a certain amount depends on your soil and your level of fitness.
Traditionally if you've heavy clay soil (and are reasonably fit) you're advised to double-dig. This basically means digging down two spades depth. see here:
http://www.rhs.org.uk/advice/profiles0105/doub le_digging.asp
Sometimes people follow a compromise of digging a single depth trench and then forking the soil in that trench to break it up a bit.
Don't kill yourself double digging the whole plot straight off - You can lay down some shallow cardbord boxes filled with soil over some of the area in the first year. Grow some crops in them the first year, then dig that patch over in the
next.