If you have moss in your lawn, you have very poor conditions to grow grass. The area is probably shady and damp most of the time.
I would start by adding 50 lbs of lime per 1000 sq ft, and get a soil test to determibne how much more lime you are going to need.
Grass does not grow well where moss exists.

Lime won't do anything to counter the shade and dampness. I know that moss is often considered a sign of low pH, but I've got high pH soil and I had moss until I was able to convince my next door neighbor that she was watering too much. Once she cut back on the water, the soil had a chance to dry out a bit and the moss went away.