You should not do that, because you shouldn't expose your Domain Controller directly to the Internet.
Though, you can do that but keep in mind the right management of the Firewall.
You need two network cards on your domain controller, one to the LAN and another one to the Internet.
You should install the RRAS role on that server (with this your server will work as a router) and in RRAS enable the feature called NAT. This is really simple and there is a lot of documentation on the web.
You should change the gateway of all your machines to be the LAN network interface card of the Domain Controller. If you have a DHCP server, make the change on it.