Before starting with CSS you should have a good knowledge of XHTML. It's a lot like HTML but a few extra rules must be followed.
Unfortunately there aren't a lot of good CSS design tools. Most of the ones that "help" with some WYSIWYG editor either limit what you can do or don't do what you want them to. And in my experience most of them don't create good code. Even Blogger here generates a bunch of crap markup.
TextPad It's free to try and you can use it indefinitely without purchasing it--Although if you like it you should purchase it to show your support. While you're at it download the syntax definitions for CSS and any other languages you want. If you want a little more there's a couple programs that may cut it.
CSSEdit2 This looks like the best one I've seen. I haven't been able to try it out much since I don't have a mac, but I like the features.
Stylemaster This one's alright, the best I've seen for Windows. It has documentation on CSS properties built in so you don't need to leave the client to look up stuff, which is a handy feature.
Web Developer Another add-on for firefox. I don't use it much since discovering Firebug but still handy at times.
Opera Another Standards Compliant Browser. Good to check your site with to make sure you're following standards. A little more strict than FF in my experience, so you can use it when debugging to see if IE is the problem or if it's Firefox.