Lalit
Security considerations aside for the moment. I'm assuming the Internet presentation is an rj45 connection ?
On the switchport on the L3 switch
int fa0/1
no switchport
ip address 192.168.5.1 255.255.255.252
where the IP address is the IP address/subnet mask supplied to you by your ISP.
There are still a lot of unanswered questions. If you just need a default-route to get to the Internet then you can just do
ip route 0.0.0.0 0.0.0.0
The major Caveat to all the above is that unless the L3 switch is a 6500 it will not do NAT so if you are trying to NAT private internal addressing to public addressing to go out onto the Internet you can't do this on a L3 switch.
Obviously there are security concerns as well. Are you putting the switch in front of a firewall. How do you propose to filter the traffic.
Jon