You're question doesn't have a simple answer.
I mean, you will need to investigate several stuff in order to get it working, as it depends on different factors, for example the video card that you have.
1. Make sure that your video card supports DRI. Most actual Nvidia and ATI cards do.
2. Make sure you have the driver (opensource or proprietary) enabled for your video card. A search on the forum might help in order to figure this out.
A good place to start is looking at /etc/X11/xorg.conf which should state the driver your video card is using. I have an ATI card, so I can only tell you that it should either be fglrx or radeon.
Also look whether you have the following enabled:
Although the above options might differ. I'm not an expert at this, but I have seen people using other configurations.
I hope this already gets you started.