The very first thing you should do is back up your data. If you don't want to read and follow the information in the links I provided above, you could download and use the
Mist app from github and use it to create a Mojave installation USB. You'll find the download link on the right, under Releases. You'll need a USB stick or disk at least 16 GB big.
Doing a clean install involves completely erasing the disk, hence my repeated advice to back your data up. That's also why I suggest using an installer USB rather than running the installation app directly from Big Sur - if things go wrong, you can try again.
There are many articles on the web,
like this one, which explain the process better than I can. If this is your only computer and you can't afford to be without it, then maybe you can find someone with experience locally to help.