newbie and no idea

A friend of mine just wrote a database for me using postgres.
I have postgres installed but can't figure out how to change the passwd
for user postgres.

[root(at)bzq-218-14-208 aamehl]# ps -U postgres
PID TTY TIME CMD
4014 pts/1 00:00:00 postmaster
4016 pts/1 00:00:00 postmaster
4017 pts/1 00:00:00 postmaster
[root(at)bzq-218-14-208 aamehl]#
---------------------------------------
root(at)bzq-218-14-208 aamehl]# ps -l -C postmaster
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
0 S 26 4014 1 0 72 0 - 2446 do_sel pts/1 00:00:00
postmaster
1 S 26 4016 4014 0 72 0 - 2693 do_sel pts/1 00:00:00
postmaster
1 S 26 4017 4016 0 72 0 - 2451 do_sel pts/1 00:00:00
postmaster
[root(at)bzq-218-14-208 aamehl]#

----------------------------------------------------------------
[root(at)bzq-218-14-208 aamehl]# createdb -U postgres nigun_test
psql: FATAL: IDENT authentication failed for user "postgres"

createdb: database creation failed
[root(at)bzq-218-14-208 aamehl]#

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Any idea??
I am also interested in getting some front end up and running.
Thanks
Aaron