Basically there is no point and click method of doing this on Ubuntu that I know of. What your effectively trying to do is masquerading so you'll find a lot of documentation if you search using that name. This can be really simple or really complicated depending on how technical you are. This document explains masquerading really well: http://www.tldp.org/HOWTO/IP-Masquerade-HOWTO/
Also take a look at this post: http://ubuntuforums.org/archive/index.php/t-25813.html
I'm not sure how you're assigning your second PC an IP address, but you might want to start by getting DNSMasq package working.
There is a lot of stuff involved but I hope this provide a good start.