1: go to File>New
2: Make sure that your preset is customer and put your width and height as the desired pixels. This will give you a working area that is the correct size you want it to be cropped to.
3: Drag a photo from your files explorer and drop it into the photoshop window. It will center itself into the working area and is adjustable from here. By holding the Shift key, you can keep your photo from getting stretched. Adjust your photo to fit within the space and press enter:
After pressing enter, your photo will crop itself to fit the size of the working area.
Save the image as a medium resolution photo—this will optimize it for the web!