Model of your new router? Your old modem will become redundant, and you will need your ISP's settings for the new router. This involves:
1) Logging into the new routers admin screen
2) Typing in the new settings from your isp
The page below will give you the required settings, just choose your make and model of adsl modem router.
The BT connection is for your telephone line
Can't help much more until we know make model etc:)