It's awfully hot and wet right now for weed control. They may have chemicals these days that don't burn or break down in extreme temperatures, but it's been raining so much it's almost a moot point. Everything would wash right down the storm drain. I would wait until next March and do the Weed-and-Feed thing. There's no sense in burning a lawn that you just planted.

Your first step is to identify the weeds. One size does not fit all when it comes to weed control. Going foward you probly should use sprigs or plugs instead of seed to fill in while useing chemicals on the lawn.
Atrazine and hormone-type herbicides such as 2,4-D and MCPP can be used to control most broadleaved weeds including clover, chickweed, henbit and thistle. Herbicides should be applied in the fall or winter before these weeds mature.
Crabgrass and other summer annuals are most effectively controlled with preemerge herbicides applied in early spring before the weeds emerge. Products containing benefin, DCPA, bensulide or simazine can be effectively used for crabgrass control when applied according to label instructions.