The concern here is that if moss is growing instead of grass, it probably means that the area is better suited to moss than grass. You can check with your local garden center for moss killers. However, to eliminate the moss and prepare the yard for grass, change the backyard’s growing conditions. Moss prefers shady, moist, and acidic conditions. Change these environmental factors, and you may be able to promote the growth of grass over that of moss. Can you trim some of the trees to increase sunlight in the area? Is it possible to improve the drainage? Does the soil need lime to raise the pH? If you can’t alter any of these conditions, grass may never grow well there. A shade-tolerant ground cover may be a better solution.