Martin, the best thing to do is to learn the difference between responsive design and adaptive design.
With responsive design every user gets the same CSS, images, etc., no matter what device they have. This is no the most effcient use of your users mobile bandwidth. With adaptive design you send only the CSS, images, etc. that is formatted to fit the particular device being used. Adaptive is generally better, but takes a lot more work.
There are a lot of resources out there, just do a google search for "adaptive design" and "responsive design" and you should see the difference.