If you have a lot of dense shade, the trees will not allow the grass to get enough sun and water, which most likely is causing thinning of the lawn. Pruning the trees will allow more sunlight to get to the grass, which needs about 4 hours of direct sunlight or 8 hours of indirect light.
Depending on the amount of shade during the day, you may want to seed with a shady seed mixture. Pure Premium Shady Grass Seed Mixture is Scotts’ best seed mixture for growing grass in shaded areas. It contains all improved seed varieties, which establish quickly into an attractive permanent lawn. This mixture thrives in areas that receive light to medium shade throughout the day. In areas of heavier shade, it may need to be reseeded annually. In areas of very dense shade, no grass will perform well.
You can learn more about seeding in our basics section or visit projects.