Confusion on 'Book new appointment'

[UK_T1G]

Hello,

I'm trying to book an appointment online

Kindly clarify my doubt - why its says - "You should not book an appointment unless you have already submitted an application and you have been informed that your service requires an appointment. "

Please do let me know - Do I need to submit my application before I make any appointment selection online? If that so please let me know where to apply/submit my application/form


Copied below from UKBA site:
"
You do not currently have any transactions.

You should not book an appointment unless you have already submitted an application and you have been informed that your service requires an appointment. "

Thank you
San

[geriatrix]

http://www.immigrationboards.com/viewtopic.php?t=103108