Welcome to the forums Philip.
If your ISP is handing out IP addresses with DHCP, make sure you check the box that gets the default gateway from DHCP. If you have a static IP, you need to add the default gateway in the routing section under static routes.
As Abel said, can you ping out by IP? That would be my first step. If yes, then check your DNS settings. You' ll need to enable DNS forwarding on the internal interface if you are using the FGT to resolve names (Network > Options). You' ll need to add valid DNS servers there as well.