the most important commands are:

cd <- change directory
cp <- copy file/folder
mv <- move file
ls <- list files/folders
cat <- show content of file
mkdir <- make directory
rm <- remove file/folder

with these seven commands you'll come very far

every command has some options too, for example:
ls -a <- show all files/folders (even hidden ones)
cp -r <- copy recursive. copies a folder with its contents.

very important is to learn how the man-pages / --help work:
man cd <- shows the manual of cd
cd --help <- shows short help

with man and --help you can look at most commands and how to use them