To start, read the official release notes.
If your install fits into "vanilla Debian plus maybe a handful of 3rd-party repos", then this guide for a simple upgrade to Debian 12 "bookworm" from Debian 11 "bullseye" can be helpful. 3rd-party repos are handled with a find command.
Note upgrade is only supported from Debian 11 to Debian 12. If you are on Debian 10, upgrade to Debian 11 first and make sure to change the security repo as per the release notes. Then once on Debian 11, you can upgrade to Debian 12.
- Check free disk space