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@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@bzq-218-14-208 aamehl]#
---------------------------------------
root@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@bzq-218-14-208 aamehl]#

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

createdb: database creation failed
[root@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