The files I need aren’t in Time Machine

In System Preferences, click Time Machine and ensure Back Up Automatically is turned on, or it’ll only back up when told to. By default, Time Machine backs up all internal drives. It can also do external ones (including USB sticks, but not network drives).

Click Options to see which drives and folders aren’t being backed up – wait a few seconds for the list to fill in. Remove any you do want backed up by selecting them and clicking the – (minus) button. To save space, add anything you don’t need backed up. Turn on ‘Notify after old backups are deleted’ to be warned when your backup drive gets full.