1. Switch your TV to the proper source/input using the TV remote or buttons on the set.
2. On the Mac, select 'System Preferences' form the Apple menu (the icon in the far left of the menu bar).
3. Click on the 'Displays' icon.
4. Click the 'Detect Displays' button in the 'Displays' dialog. If the TV is correctly connected and on the correct input, the TV should be found.
You can switch between mirroring the Mac display to the TV, or using the TV as an external display (see the 'Arrangement' tab in the Displays dialog).