[Exim] relaying question! please help

Hello
Please help me. I confused :(
I have server that connected to LAN & Internet.
I need to relay mail from local users (user@localdomain) to internet and
accept from internet mail that addressed only to localdomain.
Also I'll need receive mail from my server via internet.
How I can do this?
How can protect server from non-authorized transfer, but to keep these
opportunities.
Thank You for help,
Best Wishes,
Andrew