How do you put color or textures on a terrain?

I have tried everything such as dragging the material on the terrain and also trying to make a custom brush. Also if possible can anyone answer this question whenever I get close to my terrain it turns dark green and is split into mesh-like green squares. A screenshot of work

,I have tried everything such as dragging the material onto the terrain or trying to make a custom brush, but nothing works. Please help me this is for a school project and I only have a few weeks left.