it will kill grass, but it's not so hot on weeds and has no impact on seed growth. if you are going to 'till, you are probably better to just get out there with a rake and clean it out good before you plant. it you are going to plant starts, try some pre-emergent to help control weed regrowth. if you are plating seeds, don't do that.
i am a roundup fan, but i do believe it should be used sparingly, and i don't think i'd use it in my food garden....
those rolls of black garden mat stuff that you put down and then plant through work well. don't remember what they are called but they are easy to find.