When it warms up a bit, apply a moss killer to the area according to the directions on the package. Once the moss is dead, rake it all out to expose the soil. Next, do a pH test and apply the amount of pelletized lime that the test indicates to get the soil to a pH of 7.0 (neutral). The reason to moss got started is because the soil was never checked and lime never applied. Put down grass seed to fill in the area and keep it moist until the seed sprouts and then apply a starter lawn food to the area. In early fall, check the pH of the soil again to make sure you lime application in spring got the pH to 7.0…apply more lime then if necessary. Check the pH every couple of years and apply lime as necessary to keep the soil pH at neutral (7.0).