Step 1: Learn CSS (books, google, looking at other peoples code) Step 2: Build a dumby site that you can break and fix easily. Start small, a top head and 3 columns, no images. Step 3: Use google and the forums to figure out why what should work isnt working ( IE is going to annoy you) Step 4: Start building your site all over again. I recommend starting from scratch, dont try and convert your current site table by table.
Be patient and realize that it is going to take some time to get comfortable with the process. You are completely changing the way you know how to build websites, its going to seem dumb at first. As soon as you get more comfortable with CSS, you won't even remember how to build in tables, and will refuse to do so. When something isnt working, dont just though it into a table cell for a quick fix; hash it out and figure out how to do it with CSS.