First check that port 80 is open on your server:

# iptables-save | grep 80
-A INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT 

Switch to the zimbra user:

# su - zimbra

Turn on redirection to SSL:

zmtlsctl redirect

Now restart mailboxd:

[zimbra@mail ~]$ zmmailboxdctl stop 
Stopping mailboxd...done.
[zimbra@mail ~]$ zmmailboxdctl start
Starting mailboxd...done.

Now when your users connect to port 80 they will automatically be redirected to https://your-host-name.com

More information on zmtlsctl is available at http://www.zimbra.com/docs/os/6.0.8/administration_guide/A_app-command-line.13.13.html