One of the most fundamental principles every photographer should learn when it comes to post-processing is how to edit nondestructively. If you are learning the ropes in Photoshop, this is something you need to pay careful attention to. This fantastic video tutorial will show you to ensure that you are working in Photoshop in as nondestructive a manner as possible.
Coming to you from Anthony Morganti, this awesome video tutorial will show you how to work nondestructively in Photoshop. If you have not seen the term before, working nondestructively means you are editing in a way that does not irreversibly alter or reduce the quality of the base image. This is so important because it allows you to undo or adjust edits you have made at whatever point you would like, even years later, allowing you to reverse mistakes or change your creative direction as desired. One of the most fundamental tools for working in a nondestructive manner is layers, which allow you to essentially apply edits and effect on top of the image instead of directly to it. They are something you will use for just about every edit. Check out the video above for the full rundown from Morganti.
The most important tool for avoiding destruction of images in Photoshop is the History Brush, and while working on a picture for dozens of minutes on end you'll want to make a new snapshot from time to time in the History Palette.