Click on the crop tool. At the top of the screen, there is a bar with a number of selections for the crop tool. You'll see another image of the crop tool with a little down arrow beside it. Click the arrow. You'll see a whole bunch of aspect ratios (dimensions). Pick the one you want and it will constrain the crop to match those dimensions.
or click C key, holdt shift then click on edge of crop overlay, it should retain original aspect ratio
or look here