Firstly you will need an external screen, such as a desktop PC monitor. You can also use some modern televisons.
Ascertain which kind of video connector your laptop has and ensure you have the correct cable(s) to connect the laptop to your external screen.
Once physically connected, there is usually a key combination on the laptop's keyboard that will switch the display over to the external one. This is typically Function (or "FN") and one of the F keys, such as F5. The correct F key will have a logo looking similar to two displays printed on it.
Some laptop models will automatically detect an external screen and use it, if powered on with the external screen attached.
Consult your documentation or the manufacturer of your laptop if you need further assistance with this, or trya Google search for "how to use an external monitor with XXXXXX" <-- With your laptop make and model here in place of the X's.