You might start by defining what something is that you want to store information about, like a Person. Write the definitions out in plain language like you are trying to describe it to your grandmother. This will be the source of your Entities.
Then think of the various ways the things you have defined and write out how they can be connected or related. Use the same descriptive language. These descriptions will be the source for your relationships.
The E-R diagram is just a nice visual of your entities connected together by the relationships.