to become on isp you have to have an Internet connection, and register ip addresses that your clients can use, also dns servers for your isp will help, and probably you'll want some other things as well (if you'll be using dhcp to allow clients to get info from you, youl need a dhcp server .... etc)
also you'll need routers, to combine there connections into a few wires to save you money, and probably a few servers to serve them email, and newsgroups (if you want), and maybe a few other nice add ons (like run a file sharing server, or some game server for the LAN to help attract people)
tho this is just a rough guess at what you'll need based on things i read around while setting up my firewall rules.. I'm sure there might be other steps, or i might be wrong about some things ..
http://www.iana.org/ipaddress/ip-addresses.htm
is a place you might want to check out
