Getting Started Basic Admin

Hello,
I am very confused. I have a Debian server with PostGreSQL running. I have
everything installed and working.
Here is my wish list:
1. To do anything, I have to "su - postgres". This has no password. How do I
put a password on it. I have tried "passwd postgres wackypass" but it did
not work.
2. How do I add new users? When I add a regular user account in Linux does
it add a Postgres user automatically? I tried "createuser" and it works but
I can't login as that user in phpPgAdmin (front page login). Once I login
with the "false" security (see Question 3), I see all the users I added
with "createuser".
3. With phpPgAdmin, I can login because i turned "extra_login_security" to
false... not very secure. How does this relate to the adduser (Linux
account) and/or "createuser".

I am sorry about the questions. I have spent hours reading and piecing
together posts to try to reverse into my answers but have had no luck.

Thank you in advance,
Stephen