As more and more people are accessing websites through mobiles, I would like to adapt my sites for mobile viewing as simply and effectively as possible.

Can someone explain to me, or point me to a site that explains, how to do this? Please note that I only design sites for a hobby using HTML and CSS. Ideally, I would like my sites to be able to detect when they are being viewed through a mobile device or low-resolution screen so that the viewer is re-directed to a different version.

Is there a standard protocol for writing sites for mobiles in terms of re-direction, screen width etc?

Any advice would be most welcome.