Well, there is two few ways to do this.
- If you want to have L2 access: create bridge (apt-get install bridge-utils;brctl addbr foo;brctl addif tap1; brctl addif eth0).
- You can enable routing & NAT (sysctl net.ipv4.ip_forward=1; iptables -t nat -A POSTROUTING -i eth0 -j MASQUERADE (or -i tap1, depends on direction).
- You can set up higher levels (like squid for http proxying).
Basically, you didn't say what you got and what you want.