Share internet in Linux
Solution 1:
It seems that the best way to accomplish what you want is to route the connection to your PC through the linux system. Try this tutorial on the ubuntu community site. If this isn't what you wanted, comment.
https://help.ubuntu.com/community/Internet/ConnectionSharing
Solution 2:
To enable internet connection sharing you first have to get your internet connection working and then set up masquerading (NAT).
See the NAT HOWTO may help for how to set it up, most likely all you need is the "I just want masquerading! Help!" chapter