Here’s a quick and easy way to keep the aspect ratio of an image while cropping. You can use the image below to practise on if you like, the copyright is mine etc.,
Open the image, duplicate it and then switch off the visibility of the b/ground layer by clicking on the little eyeball. Select All, by either using the keyboard shortcut Ctrl + A, or going to Select>All in the top toolbar. The marching ants will appear around the image. Go to Select>Transform Selection and click on that to get the transform handles.
You can transform this in 2 ways, and either one is correct. Grab one of the corner handles, hold down the Shift and Alt keys and then drag the handle inwards. This way will scale from the central point in. Holding down the Shift key only whilst dragging one of the corner handles, will transform to the opposite corner to whichever corner handle you chose initially. If you use the Shift Key only, you can still move the selection to wherever you want it by placing your cursor inside the selection and moving it – just don’t grab the rotation point by mistake.
Once you have it as you like it, hit enter to accept the changes. The marching ants will still be there. To get rid of the excess around the image Go to Image>Crop in the top toolbar and then press Ctrl + D on the keyboard to deselect the marching ants and voila! Have fun 🙂