Advice from a fellow newb who made his first inroads to modding the store look this weekend.
Hopefully you use Dreamweaver MX, or feel comfortable editing in a text based editor.
Here are some useful threads,
http://forum.x-cart.com/viewtopic.php?t=74
http://forum.x-cart.com/viewtopic.php?t=1404
http://forum.x-cart.com/viewtopic.php?t=2526
http://forum.x-cart.com/viewtopic.php?t=697
http://forum.x-cart.com/viewforum.php?f=27
Read, read, search, read and then ask questions. Most of the answers to questions you may have can be found this forum, and the mods are awesome. This is one of the most helpful communties going. Just be sure to search before you ask to avoid posting redundant questions.
Don't forget to read the Official FAQ accessible from the members section.
http://www.x-cart.com/members