It's pretty easy. You just have to open yast2 and "Network Devices" > "Network Card".
Give your self an IP address (same block as your setup your dialup/file server)
Put in a subnet mask of 255.255.255.0 unless your using a larger IP range.
You probably already have the hostname and other info setup.
Then add a couple name servers from you ISP to /etc/resolve.conf and maybe even map the IP's to hostnames in /etc/hosts if you like.
Then try share the internet. ping yahoo or whatever. Good luck. I installed privoxy it's a web proxy server so all computers on the lan here have a more private surfing experience. You can rewrite content, headers, sendout custom OS/Browser identity, filter out bogus JS, pop-ups, ads and many more features. Look into this package if you want to try a web proxy server. (it comes packaged with SuSE).