You probably found this post because you're trying to crop images in WordPress' Media Library Image Editor. The button's greyed out right from the start. Oh No! How Do I Crop images with WordPress?
It's Real Simple.
You just have to click and drag on the image, the area you want to crop. Once you let go of the mouse the button becomes clickable again. Yay!
After you make your first try at selecting, you can lock the aspect ratio with the fields over to the right hand side. Select the area you want cropped and click the Crop Button (now that you can).
Make sure to hit save and enjoy your newly cropped photos!