It will probably take a full year or two to bring such a weedy area under control because there are lots of seeds in the soil that will be ready to sprout at the first opportunity. First thing you need to do is rototill or plow the area. Then water the tilled soil to encourage seeds near the surface to sprout. Wait 2 to 4 weeks, then rake out any perennial weed roots that have sprouted and cultivate again, shallowly. Plant your crops as soon as possible after you complete the second tilling.
Lay out your beds or rows so that you can easily mulch or mow between them. If you have time before you plant, bring in a load of blended compost and soil from a garden center, and spread it at least a couple of inches deep over the planting areas.
Now, buy a good weeding hoe, such as a stirrup or diamond hoe. Use it early and often, cultivating shallowly to kill any sprouted weeds without stirring up the remaining buried weed seeds. Once your soil is warm and crops are up, mulch as much of the garden as possible with straw or grass clippings. To smother weeds and build the soil in all unplanted areas, plant cover crops, such as buckwheat (in summer) and rye (in fall).