Now i'm pretty a new comer to Sinatra/Ruby/Apache but have inherited a Sinatra application to deploy.

Presently Apache is to establish to operate from the document root (httpdocs) and I have to operate a ruby application underneath a folder subdirectory for example: /httpdocs/webapp

What should i caused by understand this ready to go within subdirectory?

This link may be useful, it describes how you can deploy a Sinatra application with Apache using Passenger (mod_rack): Implementing a Sinatra Application with Apache and Phusion Passenger

Negligence particular interest for you may be the RackBaseURI option within the virtual host configuration. The state documentation can be obtained here: Phusion Passenger customers guide - Implementing Rack to Sub URI