When the weeds first emerge from the soil, till it deep; a foot is good.
A week prior to setting out plants/seeds, till it again. This will finish off any weeds/flowers that manage to survive the first tilling.
For extra measure, you can also lay down some black landscape fabric after the second tilling and cut X's in it where you wish to set out each plant. Leave it in place for the whole season.
I wouldn't use any type of herbicide/weed killer in the same area where you plan to grow vegetables...the residue is enough to stunt growth, and in a lot of cases, kill your transplants outright.