You have 3 options:
1. Most DSL providers give you a small network device which performs the functionality combined of "Router/modem/AccessPoint/4x port Switch" and initiates the PPPoE session with your ISP and also NAT.
In that case, you can just connect the WAN port of your Cisco 881G to any LAN port of that small network device and you should be fine.
2. On the other hand, you can configure that small network device as "bridge" and initiate the PPPoE session from the Cisco Router itself.
3. If your Cisco Router has a aDSL interface, you can terminate the DSL link directly on that port and you need to configure the ATM/PPPoE parameters on the Router itself.
Let us know.
Hector already gave the options you have. My preferences for implementing these options were 3, 2, 1. With 1, you should make sure to configure a static route to route internal network on the ISP-device and disable NAT on the Cisco router to avoid two NATs on the way out.