1. Make sure your set-top box is properly connected to your TV. Use an HDMI cable (or other appropriate audio/video cable) to connect the output port of your set-top box to the appropriate input port on your TV. Make sure both your set-top box and TV are turned on.
2. Turn on your Toshiba TV and use the remote control to access the TV's main menu. Most Toshiba TVs have a "Menu" button on the remote control, click it to open the main menu.
3. In the main menu, use the remote's arrow keys to navigate to "Settings" or "Setup Options". The exact name may vary depending on the model of your Toshiba TV.
4. In the Settings menu, look for and select Input Source, External Input, or a similarly named option. This option is typically used to manage input from various external devices.
5. In the "Input Source" menu, you should be able to see the different input options (e.g. HDMI1, HDMI2, AV1, etc.). Use the remote control's arrow keys to select the input port to which you are connecting your set-top box. For example, if you connected your set-top box to the HDMI1 port, select "HDMI1".
6. After making sure you have selected the correct input source, save the settings and exit the menu.
When you turn on your TV, it should display the set-top box screen by default. If the screen doesn't automatically switch to the set-top box, try pressing the "Input" or "Source" button on the remote control and select the correct HDMI input.
Please note that the above steps are provided as a guide based on the general Toshiba TV setup process. Since different models and versions of TVs may have different interfaces and options, the exact steps may vary. If you have difficulty performing the setup, you can refer to your TV model's user manual or contact Toshiba Customer Service for more detailed instructions.