In this example we want to crop the image so that we only show the tree on the left side. First select the marquee tool.

We want to crop out a rectangular region, so we'll use a rectangle marquee. Press to open up the marquee tool settings. Then select 'Rectangle' from the the 'Marquee Type' menu.

Move the cursor to the top-left corner of the region you want to crop using .

Press and hold the space bar or , then using move the cursor to the bottom-right corner of your desired region and release .

Press , then select 'Edit', then 'Crop'.

A dialog will appear to confirm that you want to perform this action. Press the 'Yes' button.

Your image has been cropped!