It seems you want to use your Wifi and Ethernet simultaneously.

In that case first thingh you need to check that your modem device has wifi capability or not. If it is, try to do second this which is just go to the you modem console and enable DHCP option.

DHCP option provide automatically IP address for both of your network adepter, Wireless and your Ethernet network adepter.

If you are still getting issue and want to more help then just go to at:

http://www.iyogi.net