You have two options really.

The first and most common is to create a different page for a mobile device and then redirecting the user to here if they are on one:

Click here to see how to redirect.

The second, which is a more recent arrival on the development scene is to create a responsive design:

Click here for more information on responsive web design.

The difference between these two are that in the first method, you can take out elements completely therefore increasing load time, but the downside is that technically you now have two different sites to develop. With responsive web design, all you are doing is using CSS to resize, reposition and/or hide elements, so load time is still likely to be the same but you still only have one site.

In my opinion, large site such as PC Advisor, which has a lot of content and different page elements, you should use a different page/site (which it does PCA Mobile Site), however sites which are blogs for example, which are just a single list of articles could use responsive design as they are just list of articles and don't contain many elements.