You did not give enough details of your problem for any of us to be of much help. It would help to know what distribution and version of that distribution you are running. You suggest that you are using a laptop with the TV plugged in via a HDMI output.
Assuming you have something like Fedora, the the System > Preferences > [Hardware] > Monitors menu will bring up your display settings. My guess is that you have your laptop display and HDMI outputs as separate displays. Make sure the Same image in all monitors checkbox is set. Then adjust the resolution to suit your needs and press Apply then Close.
As for sound, you need to set the sound preferences to use the HDMI port. Again assuming you have Fedora with Gnome, right click on the Speaker icon > Sound Preferences menu. Select the Hardware tab. In the Profiles selector, pick the one that says something like stereo hdmi, then press close.
When finished viewing you movie, you will likely have to set the monitor and sound settings back to the laptop's internal display and stereo output.