How to back up everything before formatting my PC?

It depends on how much stuff you have, but the most practical way is to use an external hard drive or a large USB drive - just plug it in, copy folder by folder (Documents, Photos, Downloads) and you're done. If you want something more automated, there are tools like Macrium Reflect or Acronis that create a full disk image, but then you need space equivalent to your hard drive's size. Don't forget to back up passwords, program settings, and those random files scattered on your Desktop that nobody remembers where they are.