"man bash" will get you started. In there, you'll see a section entitled "SHELL BUILTIN COMMANDS". Help for commands listed in this section can also be had without opening the manual page, just do "help <command>", but of course you'd have to know the name of the command first.
All of which is not all that helpful, so I looked further online. I just happened to guess an effective search term right out of the box: Guide to Linux command line.
The first result from the search site I use actually is
Be sure to check the pulldown-menu at the top of the list for subjects. Also, on the left side, very near the top, there's a link to download an offline copy.
That's enough for a start. And no one ever finishes