You have to set your default gateway to the computers in your LAN to your internal interface, which is eth0. Afterwards, if you're using firewall you must allow Web traffic. Firstly, create a service called Web through Services -> Add new. Afterwards, enter into its configuration and add TCP/any/80 and TCP/any/443. After that, go to Firewall->Packet filter->Filtering rules for internal networks and set a rule with the following data:
Decision: ACCEPT
Source: any
Destination: any
Service: Web
.
Check from your LAN a ping to your external interface to ensure about your connectivity.
Hope this helps a little.