As sanbee said if your completely used to tables and have NEVER used css before, then slowly implement it into your tables designs first...just to get used to the text and font formats and changing link styles etc.

Once you are used to a basic features of the language then you want to start learning layouts and replace your tables designs with CSS.

Personally I learnt CSS layouts in about a day on work experience I was given a project to attempt to create a CSSZenGarden project...didn't get anywhere near a submission but learnt how to use it to layout the page...since then my code has improved no end.

It may be frustrating at the beginning but stick at it. What you can do with CSS you could never do with tables, its fantastic for se friendly pages and user accessibility. Once it clicks in one palce you start to apply it to achieve different effects and your there!