You can find a ton of resources online but w3schools.com is one of the best ones to use. There are also a bunch of people on the forum who are willing to help people out too. Personally I think the best tool is good ol' trial and error, thats how I learned it.
`lots and lots of practice
`get Firebug, helps with fine tunning CSS
`if you are stuck on a bug, google it, chances are somebody has the same problem
`if you need ideas, don't be afraid to get sneaky and take a peek at other site's code. just don't be a biter and steal it
look at it, refer to it, but write your own
`be patient. (i've been going at it for about a 2 +/- years now, and I still need to peak around for some help)
`and make sure to check your CSS if its compliant. you don't want to find out from your top-notch client that your CSS may look wonderful in FF, but like crap in another browser because it was not CSS compliant
you can find the checker somewhere on the w3c page, im too lazy to look for it right now =]
I finally got around to spending some time at w3schools.com (which I've seen posted multiple times on these boards) and now I'm just saying thanks to everyone who's mentioned the site. Great learning site for all sorts of web-related skills.