Webmail users: You'll need to move your calendar, contacts, and briefcase items. These can be exported & imported and only take a few minutes. We have instructions and videos to show you how. See Migration Instructions  You'll also need to re-share any calendars that were shared. If you used filters, tasks or documents, you'll have to manually move these. We'll provide suggestions for making this easier.
Outlook users: Your ITC  will help you get Outlook switched over to use Google Apps Sync  for Outlook. As part of that process, they can import your calendar, contacts, local mail, notes, journal entries, and tasks. You'll need to re-share any calendars that were shared and recreate your filters.
POP/IMAP users: If you use some other mail client, you'll have to change your POP or IMAP settings.