give it a good rake to get out all the moss and thatch. Spot treat the large perrenial weeds with weed killer or dig them out using a small lawn weed tool. Airate the ground to give better drainage and prefered conditions for grass. Top dress the lawn with a sprinkle of top soil andsow fresh grass seed. When the grass grows and becomes stronger, begin to weed kill the lawn and you will slowly replace the weeds for grass. You may need to do this seveal times as this can not be done over-night. Wait for cooler, wetter weather before sowing.