1. "It depends".
Different organizations have different needs, there's no one, simple "right" answer.
2. In the simplest case, just:
a) Get an internet connection (DSL, Cable modem: whatever)
b) Get a wireless router with some empty ports
c) Plug the router into your internet connection, and your other PCs to the router
d) Voila! Instant network. No muss, no fuss.
3. If your needs are more complex ... then you need to do some homework. Here are a few links:
'Hope that helps .. PSM