What kind of input is it? if is HDMI then have the video input HDMI on your TV. And then press FN (function key usually on lower left of laptops) + F3 (or some other F key that has a picture of a monitor).
What kind of graphics card you using. Depending if you have nvidia (control panel or ati (catalyst control center), there should be a program that also sets the TV as primary. Also, update your graphics card driver too. Also restart the laptop if you just plugged in the HDMI.