Begin by opening your image and selecting the Crop tool
From the menu options that become available at the top left, choose the Unconstrained option to give yourself the most flexibility. Other good options are available here, so take note of those for future cropping projects.
Your menu should now look the picture below. You're now ready to drag and drop the dashed lines at the edges of the image picture to designate where you'd like to crop it.
Drag and drop the constraining box from the heavier dashes at the sides and corners of the image until you're satisfied with the cropping area. The area to be cropped is the shaded area outside the box.
When you're ready to make the crop, click on a different tool such as the Move tool. That will automatically bring up the Crop the Image? prompt. Click Crop to delete the shaded area outside the box.
The shaded area is now cropped away! Be sure to save the image with a new file name in case you need to crop from the original again in the future.
If you plan to post the picture online, use the Save for Web option to save the new image.
Photos look best in .jpeg or .png file format. The standard for web pictures is 72 dpi, so be sure to check the number in the Quality box before you click Save.