Heres a quick but I think very useful way to Crop and Compose your Images with Adobe Lightroom :
Once you have your Image open in Develop you hit the
R key and it goes into Crop Mode, now hit the
O key and select the grid you want to use theres 6 to choose from,
once you selected your grid you can press
Shift+O that will shift the grid around until you set it up the way you want to use it.
Before you apply the Crop you can press the
H key to Show/Hide the Grid or crop overlay.
Here are the Crop Overlays you have to choose from.