This video shows you clearly how to use Time Machine. Now that I have had to seriously use it I can only say – Back up your Mac! I have always used Time machine. You just turn it on and then when you plug in the external drive it just backs up. You can restore an earlier version of your Mac, or transfer the files to set up a new Mac the way you had your old one, or find individual files. It is an invaluable piece of software. The first time you use it , it can take a while because it is backing up the whole system. After that it becomes very fast and easy. MacWorld also has an article to help you with Time Machine.