First, note that you'll want to create accounts in Zimbra first before you do the migration. If you want to do it in one shot, read Bulk Provisioning.
Migrating your users means you're going to have to deal with mail, calendar, contacts and passwords. This page used to be one monolithic monstrosity -- but has now been broken down into separate pages:
- Mail Migration
- Calendar and Contacts Migration
- Password Migration
- Email Rules Migration
- Aliases file Migration
- Postfix+MySQL aliases and accounts migration
- Zimbra to Zimbra Server Migration
Please see User Migration Troubleshooting if you are having problems migrating data to your Zimbra installation.