In India, there is no good webservice that collates information of all theatres.
A quick google search also tells me about sites such as http://movies.paymate.co.in/ , http://www.ticketnew.com/ , http://www.easymoviesindia.com/ and http://www.kyazoonga.com/ who are selling movie tickets online in India.
It is always good to research properly before starting a new venture.
How to develop such web application?
2. You need to integrate it with an online payment processing service that let's you accept payments through credit cards, netbanking etc. Bookmyshow avails services of BillDesk (http://www.billdesk.com/)
3. The hardest part, you need to integrate your web based booking system with booking system of each and every individual theaters(or provide them your own system for their on the spot booking) so that your web application gets real time information about available seats and present it to users for preferred seat selection.
Depending on your budget, I would use Magento Commerece (http://www.magentocommerce.com/). I have used in the past and they have a large variety of extensions, some community driven and some commercially driven. It is written in PHP and uses the MVC pattern.
I would use the Ruby language and Ruby on Rails framework and customize the Spree http://spreecommerce.com/ E-Commerce software.