All rooms with installed data projection equipment will have a flying lead for connection to a laptop. This will connect directly to most laptops although some new machines, and in particular Apple equipment, will require an adaptor (most types are held by AV and can be booked by calling x3319).
Once connected, your laptop should automatically sense the external display and begin sending images (this works more reliably if you attach the laptop before switching it on). If not, you will need to activate the external display driver manually.
This can be achieved in a number of ways and differs between laptops. Common methods involve using the function (FN) keys on the machine. Look at the images or text on the numbered 'F' keys along the top row of the laptop keyboard - you should find a picture of a monitor or TV type screen or perhaps text such as 'LCD/CRT'. Press and hold the 'FN' key (bottom left of keyboard) and tap the appropriate 'F' key. This should bring up the display options menu which you can toggle through by tapping the 'F' key.
If this method does not work then you will need to look at your device's display settings. This can be found in the control panel (on Windows machines) or in System Preferences on Macs.