How do I start writing code for hotel management system?

At first you must think about the requirements and needed interface and other stuff like security, internet access and servers. Than you must make some architectual sketching for what I like UML use case to cover all needed functions.

After that and some discussions with your project partners that you can start to code. Remember: The real coding starts very late in the project!!!

I think you will end in some HTML spitting and transaction server back end. Like that QT backed webserver sample project.