To add a service, navigate to your Services screen by going to Settings → Services and Classes and click the button to "Add New Service":

This will take you into the Add New Service screen, this screen is divided has different fields:

In the example below I added a new service and name it "Follow up consultation", added a brief description for the service, allowed client to register for the waitlist for this service, set the duration for appointment for this service to 30 minutes, and added a buffer of 10 minutes after each appointment for this service:

After reviewing the information entered, scroll down to the bottom of the page and click "Save Service" to be directed into the Service's Profile:

Now when your clients click through your scheduler, they will see this new service as another appointment offering:

After adding a service, you may want to reorder how it is displayed in your services list or update some of the information in its service profile (like name, duration, description or price).