There are a bunch of different ways to create an ER diagram for an online movie ticket booking system.
Essentially, what you are creating is a flowchart detailing exactly what endpoints and entities will be required, from the consumer - to the potential email database, to the ticket issuing system.
One example can be found here:
Remember that an Entity Relationship model is an simple way of physically describing a computer database.
Think of data being stored in tables. Often these tables point to other tables.
For example, imagine you create a new user. That is one entity.
Let's say that user has 3 phone numbers. That is 3 data entities, all associated with that single "user" entity.
Charting these type of relationships is what ER diagrams are all about.