How to connect to the database server

Hello list.

My problem is the following: I've installed a postgresql server on my debian
GNU/linux system. I've not installed for postgresql itself, but rather because I
needed a database server for other applications that use the database for
storage purposes.

Usually, using aptitude is always a pleasure... However, I now have a database
server and I do not have any more the credentials to access the database as the
database administrator (or perhaps I have them somewhere, but I'm unable to find
them again).

My question is the following: How can I get to connect directly to the database
as the database superuser? Do I need to delete and reinstall my whole database?

I've been looking for documentation on this topic, but perhaps in the wrong
placem or perhaps in the right but failing to see the light... So if you have a
relevant link, I'm dying to hear about it.

Guillaume.