Backup ubuntu and reinstall

Hello. I'm trying to learn how to back up on Ubuntu 24.04(with a Dell laptop).
I'm an absolute beginner and can see the laptop I'm on is stalling all the time. Won't open many pages and just dies.
Thinking I need to back-up and do a fresh install.
Where can I get help to learn how to do this one step at a time?
Hope it's okay to ask these basic questions.
I read DEV is beginner-friendly with articles on installation and basic commands.
I really need this computer to work!
Thanks