Have you done any web design before? You probably need to to know at least HTML as well as CSS. A good place to start learning either is W3Schools, once you know the basics you should be able to get going, although depending on how you want your site to look you'll probably need some way of making graphics too (Photoshop, Illustrator, Fireworks etc). Sorry if that sounds off-putting, it isn't meant to be! I'm sure there's lots of people on here (me included) who would be happy to help you if you get stuck :)
A good way to get started might be to do a search for templates for whatever platform your blog is created in (blogger, wordpress, typepad etc), and have a look at how they fit together i.e. what bit of code does what. Find a template you like the layout of e.g. has the number of columns you want etc, then tweak things and see how your changes affect what you see on the screen and see if you can make something you like.
PS - this is more or less how I learnt and I now make webpages for a living (actually I taught myself CSS and HTML so I could make custom Myspace pages for myself and my friends, back in the days when no-one had ever heard of Facebook!)