I need to know how to get rid of this type of grass/weed in my lawn.
You have a mature stand of crabgrass to contend with. This late in the season, you may as well wait for it to die back, then in the spring be prepared to apply a pre-emergent crabgrass preventer. There are several products available, most of which are granular. The first application should be made when the forsythia blossoms just begin to fall from the shrubs and a second application should be made six weeks later.
You could over-seed this fall if you wish, but you will be unable to re-seed in the spring if you apply a pre-emergent.