How many public ip address that you will have from your ISP for CPE's (PC's behind cable modem).
If you have 3, then its easy..just plug it in.
If you just have one, then you need to first have a static public ip address from your ISP. That way ubr900 can run a PAT on it. You can also get dynamic ip address for CPE but it may change later so access from outside to inside will be difficult..you need to remember the ip address all the time. Static ip address make the life easier as it will not change.
First configure the cable modem for routing mode. Visit following url for that
http://www.cisco.com/en/US/partner/products/hw/cable/ps2221/products_tech_note09186a0080093d73.shtml
Now once you get ip address, you need to run PAT on it so that you can have a web server or email server on privet network behind cable modem.
http://www.cisco.com/en/US/tech/tk175/tk15/technologies_configuration_example09186a0080093e51.shtml
Just replace 827 with cable modem..Good luck