How to write custom functions.php file?

I’ve been trying to develop a theme for a couple of months now. Funny thing is, I’ve got the theme part down pretty good (especially thanks to some who have volunteered to check it out and be my guinea pig!) – but I’m having an issue with the functions.php file.

I’m beginning to think I’m jumping in over my head!

What I’m trying to do is make this theme a good one, that’s fairly “bullet-proof” when it comes to users who have very little-to-no experience with coding and stuff. I’d like to make an extra tab in the administration panel (think “Tarski”) that will allow the end user to alter the theme and do certain things with their blog without having to go in and hack things to bits. Just give them choices that they can check off or select that will make the necessary changes for them.

I’m starting to think that what I’m attempting to do is write my own plugin – which, honestly, scares the crap out of me LOL I’m a designer, not a programmer. But I digress. I still want to do this.

So, I was wondering if anyone could point me to sites/books/links anything that would give me an idea of the functions.php file and what you can do with it. Am I trying to write a plugin by doing what I want to do?

I’ve already seen (and tried to hack) the Tarski theme’s functions.php file, and I’ve also got a copy of the theme toolkit from Ozh – but I can’t really figure it out (yet). I’d love to have information on this file so I can truly see what it can do, and how I can utilize it.

Any help would be appreciated!