reseeding in sept

ok rite now my lawn is a mess its grass with lots of crab grass and some moss looking to reseed this september nnot exactly sure how  to start . should i pull the crab grass (its alot) should i put sometheng down before i seed ? do i stop mowing ?  if anyone can offer me a plan id appreciate it . im on long island and have beautiful flower gardens and vegetable gardens just need some good grass im addicted and not gonna give up!