What kind of grass do you have now? Did the builder sod the lawn with bermuda? Don't buy anything or do anything until you come back with that answer.

Is the grass dormant now?

Did you fertilize last fall?

Generally preemergent herbicide goes down when the forsythia begin to bloom in your neighborhood. Spot spray individual weeds in April. Fertilize everything but bermuda for the first time in late May (Memorial Day), Fertilize for the second time in early September (Labor Day). Bermuda can take all the high nitrogen fertilizer you can give it on a monthly basis. Mulch mow every week. Water deeply and infrequently. We can get more specific after we know what kind of grass you have.

Yes, the sod is builder-grade. Basic bermuda, I believe. I don't believe it's dormant, grass is green and looks alright visually (with the exception of the spot weeds). And no, I haven't done anything with it except mow.