Dig a weed when you see it. Don't allow it to go to seed, which starts a new weed in your lawn. Quackgrass or other grassy weeds should be dug out, removing the entire root. Then place top soil and good grass seed in the bare spot that remains. Keep this area moist until the seedlings are up and strong.
Desperation sometimes drives folks to use herbicides.
If you must, use only a systemic one that kills only the plant to which it is applied. Be cautious and use only the smallest amount possible. Read the directions before applying any chemicals to your lawn. Using too much weed killer or using it incorrectly can cause your Garden to suffer. Some herbicides can leach through the ground and kill valuable Garden plants.