Basic Terrain Question

Hi all, I'm learning how to make 3D games, but i'm a bit confused right now. I followed some tutorials for creating boxes, showing lights etc and now I would like to create some basic terrain. I can understand a bit how a heightmap works, but I don't see how to put textures on it. So if I would like to have some grass field, I need to have 2 pictures? One for the heightmap and one for the grass, both 100*100 to make it easy. But how to connect them? Do I need to take pixel by pixel out of the grass texture and point that color to the heightmap or is there another way to do this? Also for later on, I would like to make it more advance in the way the landscape can change. For example, digging a hole or something like that. Is using a full mesh for this the best approche? Oh yeah, I'm using jogl for this. So some clear tutorials for this or opengl are always welcome, but I will already be happy with an some explination to my questions :-). Thanks you!