how do i backup my computer

Back up your Mac with Time Machine - Apple Support

Read the information located in the link I provided above.

"Open Time Machine preferences from the Time Machine menu  in the menu bar. Or choose Apple menu  > System Preferences, then click Time Machine."

Vees3 wrote:

THANK YOU...one more question - if you know... do I have to back it up if I need to update my software?

You should always do regular backups and if doing an upgrade or major update always perform a backup prior.