Spray the weeds with Roundup, assuming you have no grass currently growing in the yard that you want to keep. Roundup will kill all of the weeds, plus any existing grass. Wait two weeks and spray again. Wait another two weeks and plant, if you can wait that long. Otherwise, just spray one time and wait two weeks before seeding.

Also, you may get more responses by posting your question on the lawn care forum.

Are you laying sod? There's a very good chance that if it is good, thick healthy sod, it will smother all the weeds. The only exception is if your lawn is infected with Florida Betony. I find that Weed-B-Gon is more effective that Round-Up on that stuff. And you do need to spray so that the roots will die before you lay sod. I think Betony will grow up through concrete!! It's as bad as kudzu if it likes a spot.
Frog