PC laptop users should be able to cycle through the available display options using a special function key in conjunction with the Fn button. Keep pressing this to cycle between laptop display only, TV only, and laptop and TV together. Alternatively, right-click the desktop in Windows 7 or 8 to select Screen Resolution. From here you'll be able to manually detect and select your TV's display.
MacBook users can configure the display via System Preferences: select Displays followed by the Arrangement tab (click Detect Displays if it's not present). Tick Mirror Displays to put your TV perfectly in sync with your MacBook's display as opposed to acting as an extension of it.
Can I connect my laptop to my TV without wires?
There are plenty of options for wirelessly beaming video from your laptop to your TV. The most expensive options involve purchasing a Wireless HDMI sender and receiver kit such as ZyXEL's Whd6215 four-port Wireless HDMI Kit, but this is more aimed at wirelessly transmitting between different rooms.
It's actually more cost-effective to investigate streaming media wirelessly over your network to your TV via DLNA/UPNP. Most modern Smart TVs are capable of directly accessing media in this fashion - all you need to do is turn your laptop into a media server following our advice in this article - but even if your TV doesn't sport Smart capabilities, the cost of buying a suitable set-top media player like the WDTV Live Streaming Media Player is much less than going down the wireless HDMI route. It also allows you to add other smart features to your TV like BBC iPlayer and YouTube.
Apple aficionados may prefer to go with the Apple TV, but while it works flawlessly with iTunes and your Apple mobile devices, it's less flexible with non-Apple equipment. Unless you're wedded to all things Apple, we'd suggest investigating the UPNP/DLNA solution instead.