Find here some references to general documentations.
You can use e.g.
to display the man pages of
cp. An alternative is to install tldr and look up information from their pages.
If your are using a dedicated programm, it might have some built-in syntax like e.g.
git help commit
additional of course to its official documentation.
Detailed information (german only) can be found in the ubuntuusers-wiki.
We also like to use the archlinux-wiki. This is usually very detailed and might be more suited for advanced users.
We can not cover all problems in this Knowledge Base. But we can point you to your favorite search engine and this might bring you to pages like stackoverflow.