For me, nothing like the old WordPad, or at least NotePad. It lets you see the basic Raw code and learn from it as well, that way, no matter where you are (away from your computer) you can edit anything on the fly, as soon as you know what your doing.
As for How, in the Xcart Manual there is a VERY, VERY basic tutor on TPL(good for a start point):
http://www.x-cart.com/xcart_manual/index.html
Or go to Smarty website for more information.
And regarding PHP, well it takes time, but maybe a good start point would be:
http://www.developer.com/lang/php/article.php/900521
( Learning PHP: The What's and the Why's )
Good Luck.