• SH - Bourne Shell,
• BASH - Bourne Again Shell

Once you figure those two out Linux becomes ridiculously easy afterwards as those will be your standard shells to use which provide a set list of basic commands like cd, nano, cat, ls, rm and so on, then you want to figure out which package manager your distro uses like Apt, Dpkg, Yum, Pacman so you can install software from their respective repositories.

Then you figure out which software is Cli (command line) based and which have actual user interfaces, Cli based software will likely introduce new commands to your shells.