Creating the ER Model after the creation fo the database

Question

I've created a program runnning on the web (html) about booking tickets on a cinema theater.

I now want to create its ER Model.

As far as I know, the logical sequence of actions is:

1. Create the ER Model
2. Generate the Physical Model
3. Create your Database

So now, when I try to create the ER model, the relation between my entities doesn't seem right. On the other hand, if I add more entities like customer or admin, then that means that I have created more tables for my database that I don't need.

Anyone can help me in this one?