First I looked through the free templates to see what they're comprised of, then I'll tweak and work with them until I can get the code memorized(or partially, at least). That's been my comfort zone so far and I haven't ventured very far from changing box sizes, putting in music players, and changing the colour of every single thing at least once.
This is the hub I use to get to different CSS help sections.
Hopefully someone with more experience with CSS will stop by and offer more help than I.

neopets honestly. or that's where I first started.
nostalgia...

Awake \o/
I, too, started with Neopets. I found some basic tutorials via Google and used Neopets to play with what I learned. I made guild layouts and learned the basics of div boxes, text formatting, and what-have-you.
That said, Neopets was more a medium to play in than a true teaching source. I used to have an entire bookmark folder dedicated to an assortment of websites I'd found with snips of HTML and CSS codes. Figure out what you want to do, and then use Google to find a basic resource to start with. From there, it's mostly tinkering and self-teaching. TEF is a different beast because of its forum layout, so not all codes work like they 'should' when used in TEF vs. a blank webpage.
I don't have my old bookmark folder any more because it was on an old computer, but here are some links I've found with a quick search:

1
2
3
4
5
^ that last one looks pretty cool o3o Might have to play with that

Just keep in mind that most tutorials out there are going to be geared towards either specific pieces of code (ie, "how to use a div box") or writing an entire page or site/working with a blank canvass - which TEF is not, and you may need to adjust things here and there or ask someone more knowledgeable when you get stuck.