Let me first say that a newbie shouldn't worry a lot about adding modules and themes at first. Work on the basics of your site first, then worry about add-ons.
Themes are largely a matter of taste. For example, I have no idea why anyone would use a "fixed width" theme, but lots of people do. One nice thing about themes are they are pretty much independent of your content (later on you can look at the many submissions that are dependent on content).
Contributed modules are ways to add or extend functionality of your site. The only module I, personally, consider necessary is the Nodewords (a.k.a Meta Tags) module; in my opinion, it should be promoted to "core" status. This one allows you to add the "content," "keywords," and "robots" meta tags to your pages. This is useful if you're interested in your search engine rankings. You will also find that many contributed modules also require the Views module; I go ahead and make that a standard one for my sites.
I am sorry, but when I realized I needed to move my resume here, I had to change the basic Bluemarine theme a little. What you see here (other than the logo picture) are very small changes to the style sheet.
Resist the temptation to add every module that looks like it might be useful someday! If you don't NEED it today, don't install it. Why? There are several reasons for my suggestion: 1) Not all modules play well with others - most do - research the forums and support requests. 2) I'd be willing to bet that you won't need all of them, and removing them can be problematic, especially the access control modules (worry about security later in your project). 3) Not all modules will make it into the next release of Drupal, and some will be delayed after the release - research their history, research their bug reports - be sure you want to invest in that module. 4) After your understand Drupal a little better, you may realize you don't really need that module.
Do keep a list of things that you might want or that your current site is not doing yet. Prioritize it. But don't be completely hung up on following the priority list strictly. Some times item #2 will be much easier to do after you've done item #7. (Okay, technically that means you mis-prioritized item #7, but who's counting?)
themesfolder. If you are not running multiple sites, this would be