Postgres on Unix/Debian - seeking "dummies guide"

Hi all,

Utter newbie question. There will be no single answer - so if anyone could
just point me in the right direction with a "recipe" I would be most
grateful.

In summary ...
===========
- for last 10 years been Windows based with a java desktop accessing
Postres with no issues at all
- currently trying to port the app to Debian
- it's 20+ years since I last used *nix
- After several days - I still can't get the java app on (say) Debian user
"damian" to access the db that is running as Debian user "postgres".
PgAdmin3 works fine from "damian", but not the java app with the same user
& pwds.
- seeking any beginner clues / recipie for the most basic setup to get it
working
- not looking for "best practice", just an effective guide to what would
work for initial dev phase.

In more detail ...
=============
Our distributed Java desktop app uses Postgres as its DB, and has been on
Windows for many years, and that process (dev to deployment) is completely
solid. In the target industry all customers must be on Windows, hence so is
our dev environment.

My current task is porting to Debian - but it's my first Unix since 1993
when I played on Solaris - so my familiarity has long gone and I'm fumbling
the basics. For the life of me I can't get the app to get access to the DB.

For these trials I'm using Debian on VirtualBox. I don't think that would
be relevant.

I *think* it is only a matter of users, permissions, roles, groups, pwds
etc - but after following 10+ tutorials and 2 days of trying I am
officially struggling. Particularly roles and groups, these are in the
tutorials, but

I just want to get the most basic case working, and I can scale from there.

2 Debian users / passwords
- postgres / pgpwd
- damian / mypwd

DB added to PG
- db name: mydb
- db owner: postgres

PostgreSQL installed for user postgres - all good

for user damian
1. PgAdmin3 installed and connects to DB nicely
2. Java app installed and running properly in every way except it is denied
access to the DB

Even if I use dbuser: "postgres" and dbpwd: "pgpwd" the logs say ...
<logs>
Connection matched pg_hba.conf line 92: "host all all 127.0.0.1/32 md5"
postgres(at)qdb FATAL: password authentication failed for user "postgres"
postgres(at)qdb DETAIL: Password does not match for user "postgres".
</logs>

- What username and password is PG expecting?
- is this expecting a role to be created or something?

Any clues would be much appreciated.

Thanks,
-Damian