Emacs
Emacs is a powerful text editor, that has extensions for all sorts of things, like highlighting syntax of programming languages and reading email. It can be run as an X-Window, which allows use of the mouse, or with the -nw option, to specify no windowing to work correctly in a simple text window. To create or edit a text file named abc.txt, simply type:emacs abc.txt
Typing characters will enter them into the file. To exit Emacs, type:C-x C-c
(Read: Control-x, Control-c: Hold the Control key down like a shift key, then hit x, then again with c.
Emacs commands and cursor movements are driven by Control and Escape sequences, and by (optional) mouse clicks.
Becoming adept at a text editor is important skill for mastering Linux, and Emacs is a popular choice. Emacs tutorials can be found here, here, and here. Also see the official Emacs manual.
There are, of course, other text editors for Linux, such as vi.