Read the manuals, or a good book on Unix and the manual pages (type man man). There is a lot of GNU Info documentation, which is often more useful as a tutorial. Run Emacs and type F1-i, or type info info if you don't have or don't like Emacs. Note that the Emacs libc node may not exactly describe the latest Linux libc, or GNU glibc2. But the GNU project and LDP are always looking for volunteers to
upgrade their library documentation.
Anyway, between the existing Texinfo documentation, and the manual pages in sections 2 and 3, should provide enough information to get started.
As with all free software, the best tutorial is the source code itself.
The latest release of the Linux manual pages, a collection of useful GNU Info documentation, and various other information related to programming Linux, can be found on metalab.unc.edu/pub/Linux/docs/man-pages/.