1. Make sure you've got an aerial plugged in.
2. Press the 'Source' button on the remote control. (It's sometimes marked with an arrow pointing into a rounded rectangle).
3. Scroll through the options to ensure that you've got something like 'DTV' (digital television), 'RF' (radio frequency) or 'Ant' (Antenna) selected. Then exit that screen.
4. Press the 'Menu' button and look for an option to 'Auto-tune' or similar. The TV will then scan for available channels.
If you need further help please tell us the exact model of your Toshiba TV. (It'll be on the back of the set).