The best defense is a good offense. With respect to lawns, a thick, vigorous turf helps weed control by out-competing undesirable, opportunistic plant species. Once you have installed sod, maintaining a weed-free yard can be accomplished by properly watering, fertilizing, and mowing the turf, as well as applying herbicides as needed.
If a few weeds are noticed, manual removal should suffice. If your weed problem is more widespread, herbicides can be applied that are specific to the invading plant(s). Proactive weed prevention for crabgrass, for example, can be applied in April (some crabgrass pre-emergent herbicides contain broadleaf weed control as well). ALWAYS READ AND FOLLOW THE LABEL FOR HERBICIDE APPLICATION DIRECTIONS AND RATES! An excellent tool for identifying weeds is the Virginia Tech Weed Identification Guide. If you are not comfortable with weed identification or weed control application, you can hire a lawn maintenance provider to perform these services for your yard.