When I started, I read around a bit and looked at the files.
I believe I'm fairly proficient in skinning, so I'll tell you what I know:
1) The firebug plugin for firefox can be equated to finding out how to travel faster than light. AKA, its epic and you need it.
2) Develop your site using firefox as the primary browser and then once finished go back and check it in other browsers. You can also drop support for IE6 as major sites have as well, so its not a small drop of sites forcing people to upgrade.
3) Keep debugging console active while developing.
4) When you can't figure out which template you're needing to edit, turn on webmaster mode and then highlight the .tpl names in the debug window and then the content inside that template will "pop"
5) If you're not comfortable with starting from a base skin, then you should buy a template from someplace that sort of looks like what you want and then customize from there.
6) Brush up on CSS if you're not fluent in it. For starting out, all you'll need to really know without looking is padding, margin, color, background color/image, positioning.
7) I would suggest using an IDE or text editor (I love notepad++) that will highlight and show HTML easier. This is so you know if you've closed all your tags and such.
When working with smarty it's a good idea to read a crash course on it.
http://www.smarty.net/crashcourse.php
The basics are this:
Smarty's delimeters are { and } so any inline javascript should be surronded by {literal}{/literal} tags so smarty doesnt try to parse it.
Variables are called like {$variable}
Arrays are called like {$variable.name2.name3}
Operators are eq,neq,lt,lte,gt,gte or the standard php type of == >= <= etc
9) You will need to clear your xcart cache frequently.
www.yoursite.com/cleanup.php
10) If you need help, post on the forums in the right area.
11) Search is a better button to use than the new topic button. Use the search before posting
Right now I'm in the process of updating one of my mods to work with 4.2 and 4.1, but if you need to know a little bit more about smarty {if}{elseif}{else} statements, lemme know and I'll give you a link to an unfinished article I have that relates to X-cart.