Hi Laura,
The easiest way to do that is using the VNS wizard by selecting "New..." in the Controller VNS menu. Select "Start VNS Wizard, and create a Captive Portal topology, and in the next screen choose GuestPortal from the Authentication Mode drop down. Continue through the wizard which will step you through setting up the Topology and subnet that guest users will be on, and add in what AP's you want to broadcast the SSID.
After saving, this will set up a default non-authenticated and authenticated Role for the guest network. To restrict users to the internet only, you can go to the Role > Authenticated Policy > Filters and add a Deny statement at the bottom, and then add in Allow rules for DHCP, DNS, HTTP, HTTPS above that.
If you would like to use an existing Topology for guest users, but still restrict them to the internet only, you can change the Contain to VLAN be any L3 topology that is configured.
To configure guest user login access, go to WLAN Services, select your guest WLAN. Then select the Auth & Acct tab and Configure to add user names and passwords, time of day restrictions, etc.
Hope that helps.
Regards,
Jason