Well you need to know at least a little bit smarty to be able to understand the templates -
www.smarty.net
Also you need to know css and html to be able to move elements around and style them the way you want. Firebug addon for FF is necessary tool when working on the templates. Read this -
http://forum.x-cart.com/showthread.php?t=21529 - while it is for older versions the basics still apply. Remember the templates for customer side in 4.3.x are in skin1/customer