These how to may help,

How to enable windows sharingHow to enable Windows sharing. One of the reasons to create a workgroup or domain network is for all computers on the network to access the network ...
     www.howtonetworking.com/Networking/enablesharing0.htm

     Sharing between Mac and WindowsSharing Printers and Files over a private network · Internet access sharing · How to access network resources · Troubleshooting windows sharing issues ...
           www.howtonetworking.com/mac/mac&windows.htm
   

Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net 
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com 
 Dear network experts,

 I need to have a few computers to share the data and Internet
 connectivity.

 How do I do it if I have:
 - Mac computer & 2 Windows laptops
 - Cable modem
 - Wireless Airport router (only Mac can use it)

 It is a simple network just to store my data at one place (under
 100GB), back it up and access remotely from outside of the office area.

 Please advice!
 Thank you.