Can't change to user "postgres"

Hi,
these are my first steps with databases,
and i can't get postgres to work.
I followed the instructions concerning debian given for example here
http://www.postgresql.org/docs/7.4/interactive/installation.html ( in
the comment section )
which is pretty easy.
But i can't get beyond the second step, initializing the database
I can't change, that is "su", to user postgres.
If i try it ( "$ su postgres") as su/root i stay su/root.
If i give him a password and login as me(at)mycomputer, the same outcome
- the user stays the same.

I've read somewhere that the userid desired ( by the script ? ) is one
below 100, more exactly 26, which is taken on my system by some other
group. The userid of the user "postgres" is now 109. Might that pose
some problem ?

I would be grateful for some hint.

Thx
martin