Depends what you want to do. One way if you are going to turn it in to a garden is to till the whole area. Water it and wait a couple weeks for all the weed seeds you exposed to sprout, then till it again. Then plant right away or if you aren't planting, cover the whole area with a good layer of mulch (straw, cardboard, newspaper, etc).
If it is a small area, you can just turn it over with a shovel and or dig weeds out.
Otherwise you can solarize as imafan suggested (which takes a couple months) or you can spray everything with horticultural vinegar which is an organic weed killer.
Or if you are going to do raised beds, you can just lay down several layers of cardboard and/or newspaper right on top of what you have now, without worrying about killing the weeds. Water before laying the cardboard/ mulch down and then water the mulch well. Then put down 4" or more of good topsoil/compost and plant in to it. The mulch and soil smother the weeds under it and then the mulch breaks down in the next months, leaving you with a nice deep weed free planting bed.