Till the crab grass in and at the same time amend the soil as necessary and then seed. Your Cornell Cooperative Extension Service will have information about getting lawns reestablished and they will tell you now is a good time (from August 15 to September 15) to that.

kimmq is kimmsr

If I till the crabgrass will.dormant seeds come back in the spring and end up withers the same issue next summer?

You will get crab grass seeds no matter what. Tilling the grass in can bury the seeds deeper so those won't germinate, but birds and the wind carry them around and plant them. The best defense against having crab grass growing is a good thick turf.

kimmq is kimmsr

To prevent crabgrass next year, apply preemergent in early spring when the forsythia are in bloom and then at regular intervals. This will prevent the germination of new weed seeds. I use a product called Dimension (dithiopyr) made by Lesco, which is relatively long lasting (3 months per application), but there are a number of other products as well. They are often sold as "weed and feed", though for the first application in spring I like to use something that doesn't contain fertilizer as it's best not to fertilize in early spring.