What part do you have questions about. Do they have active directory yet? How big are they? Are they prepared to isolate the exchange server from the internet using a firewall and email relay, (highly recommended)? Do they require clustering to accomodate failover?
The internal network shouldn’t require changes if it is a sound design. On the other hand, I wouldn’t even consider installing exchange unless it is well protected from the internet.
As you may guess from these questions, your request covers a lot of territory. I suggest you start by reading the documents at this link:
When you have specific questions, post them here.