I can do a basic HTML site although not an expert by any means. I have heard that CSS makes changing things on the site a lot easier, though, so I would like to learn how to create my own. Did you find it difficult to learn? Can anyone point me in the right direction for any good online tuturials?

No, CSS is not hard to learn but, it does take time to master. You'll catch the basic concepts in a day or so but, because you can manipulate the design in so many different ways, it will take a while to master them all.
You'll need to get used to previewing the site in different browsers because what looks great in FireFox may not look so good (or even work) in IE and vice versa.

You can find just about anything you need to know on CSS at W3Schools.

It is a whole lot easier to change a web layout if using CSS - believe me, been there, done that got the t-shirt! If you use external stylesheets, you edit the single page off CSS, and instantly you have new layout and/or colour scheme.

Additionally, if it is this aspect of CSS that makes it of interest to you, the php include code will be an asset - works in a similar fashion to CSS in that you can put all of your menu links into the php include page, and if you wish to add a link, you edit the single page and the changes show throughout your site.

My favourite CSS tutorial site is http://www.html.net
Favourite CSS code library: http://www.dynamicdrive.com/style/
Great site for CSS navigations: http://css.maxdesign.com.au/listamatic/

Good luck!

click below links for CSS and HTML tutorials

http://www.daniweb.com/forums/thread128697.html

Thank you all for the very helpful responses. This thread definitely contains a wealth of resources and I've bookmarked each one of them. I'm determined to learn the basics, at least, and then over time I'll master all the intricacies.