How can I mirror my MacBook Air screen to my lg tv screen

About AirPlay and Airplay Mirroring

AirPlay Mirroring requires a second-generation Apple TV or later, and is supported on the following Mac models: iMac (Mid 2011 or newer), Mac mini (Mid 2011 or newer), MacBook Air (Mid 2011 or newer), and MacBook Pro (Early 2011 or newer).

Several Apple Articles Regarding AirPlay

1. Apple TV (2nd and 3rd gen)- How to use AirPlay Mirroring
2. How to set up and configure AirPort Express for AirPlay and iTunes
3. About AirPlay Mirroring in OS X
4. Troubleshooting AirPlay and AirPlay Mirroring
5. Using AirPlay

You can, if you have something like the Apple TV. Apple support document:

Use AirPlay to display video from your Mac on an HDTV - Apple Support