In a terminal window:
sudo nano /etc/xdg/lxsession/LXDE-pi/autostart- Then append the following to the end of the file and save:
/usr/bin/chromium-browser --kiosk --ignore-certificate-errors --disable-restore-session-state https://www.whateverwebsiteyouwant.com - Reboot
Note: You may get a warning/error message about Security Settings when using –ignore-certificate-errors switch. You can simply remove it and try.