Whether you would like for new subscribers to enter their information first or have them select a service before that is completely a matter of preference, because you can choose how your online sign-up process flow will go.
isp config - subscriber portals - online signup
To set-up the online sign-up process flow that you would like new subscribers to follow, open the ISP Config window, go to the Subscriber PortalsOnline Signup panel, and then the Online Signup Flow section.
If you would like to have new subscribers enter their information first before choosing a package to subscribe to, select the User Information → Select a Service → Billing Information → Confirmation (User Information first) option.
isp config - subscriber portals - online signup - user information
On the other hand, if you would rather have them select a service first before they enter their information, select the Select a Service → User Information → Billing Information → Confirmation option.
isp config - subscriber portals - online signup - service selection