The obvious answer would be to pull any visible weeds, either rent a tiller, or break up the soil by hand with a pick axe and shovel to prepare your soil. The tiller will break up the soil, and mix in the weeds or whatever was growing there previously, but the more stubborn weeds will come back.

There are alternatives that are just as effective, and much easier. You could build a raised bed garden for instance. This could consist of rail road ties stacked one or two high in a square or arbitrary shape, with the center filled with dirt to become your new garden.

Another option would be to lay down some thicker black plastic (6 mil) to use as mulch. It would be desirable to break up the soil prior to doing this. This would need to be done several weeks prior to planting to be effective. But, given sufficient time, this would kill all the weeds, and provide a viable mulch for your garden that will benefit your garden plants as well. It conserves water, and increases plant growth by concentrating the carbon dioxide in the soil around the base of the plant, where it can be utilized. It also keeps down the weeds, which is good for both the plant and your back.