Are you perhaps talking about turning on AutoSave? If this is what you are wanting to do, realize that AutoSave will not work until after you save an initial copy of the notebook - I wished it did, but it doesn't. Also AutoSave is not supported when working with Cloud only stuff, ie only when the notebook is a local copy on your computer. To turn it on use the Options Inspector and set the "NotebookAutoSave" setting to True (the default is False) ... if you're not sure how to use the Options Inspector click on the Preferences item in the Wolfram menu at the top left of the screen and then click on the "Advanced" option on the bottom left, then click the "Open Option Inspector" button in the center of the Preferences window, and then search for "AutoSave" in the GlobalPreferences group of settings, and then change it from the False setting (the default) to True - and don't forget to save it the first time. After that it should save everything you do.
Think it might be a very good idea to get into the habit of saving your work manually when you first create a new notebook. Just click the File/Save menu item and provide a name and location for where the file is to be saved. This way when AutoSave is enabled, your work will have been saved already should a computer or Mathematica crash occur, and you don't risk loosing everything you've done.
You can also set an explicit time if you wish - see the SetNotebookAutoSaveTime documentation for more information.
Good luck...if you wish more info please ask...
-bob
