To start the web browser using autostart do this:

Open the autostart file using the Nano editor:
nano /home/pi/.config/lxsession/LXDE-pi/autostart

Add this to the end of autostart:
@chromium-browser www.raspberrypi.org

Save and exit by typing CTRL x then answer y

logout and back in to desktop or reboot

There are several options you can do such as fullscreen or kiosk modes etc.

For full screen mode:
chromium-brower --start-fullscreen raspberrypi.org

Use F11 to toggle between full screen and normal

For Kiosk mode:
chromium-brower --kiosk raspberrypi.org

Like full screen but user can't use F11 so can't change to different website.
Can only use alt-F4 or ctrl w to quit browser

Substitute any URL you want to use.
Specifying more than 1 URL will open each in a TAB. Use Shift-CTRL-Tab to switch tabs.

https://www.raspberrypi.org/forums/viewtopic.php?f=91&t=219952