You can kill the ants with Bonide Ant, Flea and Tick Killer. Crabgrass requires the use of crabgrass preventer for a few years to prevent the crabgrass seeds from germinating. It is a little late to do that this year since the seeds have already sprouted. You can use a crabgrass killer spray but that is not as effective. There is no real “forever” solution to these problems.
HERE’S a link to all about starting grass from seed. The blog post is from last August but the procedure is the same. Get to your lawn seeding right away. The longer and hotter the days become, the harder it is to keep the seeds watered and moist.