A very important one besides backing up your files is that you should more or less stop searching the web for applications to download and install. You should first try to use your package manager (read the wiki of your distribution to know which one it is and how to search for and install software with it). If you miss some applications from Windows, you could have a good chance to look for alternatives at sites like www.opensourcealternative.to
Do not fear the terminal too much. I know, Windows tried to make it as dreadful as possible to use, but if you do not run sudo rm -rf / (deletes your whole filesystem), there is not much that can go awfully wrong. And you should only execute commands you understand. If something does go wrong however, it is a good thing to have a backup ready (I would recommend Pika Backup or Vorta, both based on the great CLI application borg)
For things that are not explained in the wiki of your distro, you have a pretty good chance to find a good explanation and even troubleshooting tips at the Arch Wiki (e.g. I use Void Linux and still search for most things at the Arch Wiki). It is also good to read some parts of the manual pages if you did not find enough information at the Arch Wiki (the command man is your friend) and the software’s wiki page if it exists.
If you ever find yourself in the editor vi or vim, type :q and press enter to quit until you feel the need to potentially invest a lot of time in learning vim movements to increase your editing speed and you never want to go back. Use nano, micro or a graphical app at first instead and keep using it if you are not someone who edits text a lot.