And that's an excuse why? 😉
It's so simple:
(1) Get an external harddrive (it should be bigger than the drive you want to backup and it must be bigger than the data you want to backup).
(2) Launch the System Preferences and select Time Machine from the list.
(3) Select your external harddrive as backup drive and move the slider from Off to On.
There is nothing more that needs to be done, Time Machine does all the rest automatically.
Do not do it manually, because chances are, you'll forget to do it on a regular basis.