Go to the Zimbra Admin UI and on the Global Settings/MTA tab you'll need to enter the details for your ISPs server in the Network/Relay MTA for external delivery field. Save the setting and that should do what you need. I am assuming you have a recent version of Zimbra (that filed is fairly new) and that your ISP doesn't require authentication.
If you need to Authenticate against your ISPs mail server just follow the instructions here:
Outgoing SMTP Authentication - Zimbra :: Wiki
You'd have to look at your ISPs help pages, if they have any, for details of their server names.