Which point are you getting stuck? What is the error traceback? What installation guide have you followed?
Refer to the above point!
Did you choose any of the tried and tested methods in the Wiki?
Are you referring to the business domains in the setup wizard? A simple answer is that the domain you select will give you the icons on your Desk accordingly and you will see those modules in the sidebar navigation.
You can select multiple domains.
You can find the answer here:
Or you can participate in any of the multiple postgres threads.
PS: there may be good news with respect to using Postgres in Frappe originating from this thread.
If you be more specific about what problems you are facing, you will get better answers. Vague questions will invite further vague answers and the loop will continue 