Working on Web service

Changing my web service so that the booking process is done internally instead of through the outside web service system.

Not a clue on how to start. 

My web service as it is works real well but must be changed so that it calls our internal booking functions to generate a new booking id and email from our own system.

This sounded easy until I had to call the original web service to get the details of the booking.

Now to add to it, email must be totally changed so that it has content from the service results in it instead of the existing structure used by all email.

How do I do that?

Lord if this is you sifting me, I'm having trouble seeing the light of day!