I have 100 pcs in my network and I want to share Internet in network. So how it is possible? By using router or making proxy server?

My first choice would be using a router, however a proxy server might be more appropriate in some situations in addition to a router. The typical way a router is used is to terminate the Internet connection. One or more WAN interface cards or modules are installed in the router for the specific types of Internet connections that are provisioned. The LAN or Ethernet port on the router is connected to an Ethernet switch which all the PCs are connected to. The router is then programmed to route network traffic from the LAN subnet to the WAN subnet as desired. The router can also usually be programmed to provide firewall functionality by inspecting the traffic passing through it and denying access to hosts on the LAN from the Internet. Routers also commonly provide Network Address Translation (NAT) to translate a private IP addressing scheme used on the LAN to one or more public WAN addresses.

If this is your first time setting up a network, I strongly suggest getting knowledgeable assistance with the router programming. Done incorrectly, your network may be open to the world or its security easily compromised. Additionally, Cisco and other router vendors have configuration guides available on their websites to assist in router deployment.