Before you even begin to extend Emacs, it's already the highest-function text editor there is. Not only can it do everything you'd normally expect (formatting paragraphs, centering lines, searching for patterns, putting a block in upper case), not only does it have advanced features (matching braces in source code, employing color to highlight syntactic elements in your files, giving online help on every keystroke and other commands), but it also performs a host of functions you'd never dream of finding in a text editor. You can use Emacs to read and compose email and to browse the World Wide Web; you can have it run FTP for you, transparently making remote files editable as if they were local; you can ask it to remind you about upcoming meetings, appointments, and anniversaries. As if that weren't enough, Emacs can also play you in a game of Go-Moku (and win, more than likely); it can tell you today's date in the ancient Mayan calendar; and it can decompose a number into its prime factors.
Emacs is powerful, self-documenting, customizable, and extensible editor. When you consider that it is free and readily available on Unix systems (and free and downloadable for Windows also), it is hard to beat. While it is not the easiest editor in the world to learn your way around, I think you will find that the rewards easily justify the effort, and not just for the Go-Moku.
You can easily find any number of online emacs tutorials. Note also that emacs has its complete manual available on-line in Info. After you understand the basics, try typing C-h i (while holding the control key down, push h, then release both and press i) to get started with the on-line documentation.