you need a tools collection to deal with XC: PHP, MySQL, CSS, JavaScript, JQuery, Smarty, XML, JSON, own XC functions.
the nice part of XC is the programming part is separately than design part. if you are not a programmer, you can deal only with smarty/html/css for different customization.
webmaster mode will help you identifying the templates and variables which are available for different pages. inspect code in browsers will help you see information about css, html.
you need also to know how to look over a code inside the whole XC directory. I do recommend Sublime Text 2 as default editor.
last but not the least make an effort to read the User Manual. just generate one from here in PDF:
help.x-cart.com