Welcome, davisclan, to the WP community.
Just to get you started, I would suggest that you poke around some of the sites built by community members. There is now a new feature on the forum, in that any registered member that has supplied a link to their own site has their user name hyperlinked, so you can easily find lots of interesting sites from there.
The most basic changes to the WordPress default template are made with the INDEX.PHP page and WP-LAYOUT.CSS file. There are lots of great example templates at Alex King's website as well. Find a big stack of them at his site. You can learn a lot from dissecting those templates.
You have a lot of options for functionality and such, as well as a growing batch of hacks, tweaks, and plug-ins for extending the base functions of WP.
This will get you started. Don't be hesitant to post questions of all sorts here in the forums. Try to be as specific as possible about problems, and supply version info and a link, if possible, to the problem page. There are lots of great folks around her who will help out.
Most of all, have fun!