It can get a little complicated, but it is easier then upgrading from an older version of Exchange or migrating from a different email platform.
First make sure everything is working properly in Active Directory. Then figure out what server you are going to install Exchange onto (existing server or new server). Does the server meet the requirements? Will you be enabling internet email? If so, who has the ability to create the MX record? What type of limits will you be setting on the mailboxes? What anti-virus and spam filter will you be using?
1. Get a big server.
2. Insert Exchange 2003 CD.
3. Follow prompts.
Seriously, I would recommend buying a book, either one of these would be good:MCSA/MCSE Self-Paced Training Kit (Exam 70-284): Implementing and Managing Microsoft Exchange Server 2003Learning Exchange Server 2003
There is no "Step 1,2,3" to setting up Exchange. You need to do the research, lay the groundwork, and really understand what you are doing.
If you have more specific questions along the way we'll be glad to help. Good luck.