Can't tell you everything you need to know, but you should look up some information on:
Active Directory and user accounts.
Security and permission settings
You need to know how to make PC's part of a domain.
You need to know about sharing drives.
Unless you want to run ISA Server, then one network card is fine. Recommend you have a modem router, otherwise you have to set up LAN proxy settings
If you want each person to have an account to login, access all their documents etc. You need to look at Roaming Profile, personal drive mapping, and probably group policy folder direction functions.