Small business network setup

Hello, I am setting up a small business network. I have a server I use ubuntu 11.04 server operating software on it and I have 12 PC's which use windows7 in the office, the servers and the computers are connected to a switch. I need to connect the computers to the servers, to make a network, for emails, file saving on the server, etc.
As I understand I need to install server software on the server, set it up, then install client server on the computers and connect to the server and that's it? And it should be VPN software?
Please could you correct me if I'm going the wrong way. And tell me what I need to do.

Thanks
Lukas