changing the password of postres

Dear my friends...

I am using SuSE 9.1 and postgres 7.2.x .

Each time if I want to connect to postgres with
postgres I do like this:
"
patrixlinux(at)patrix:~> su
Password:
patrix:/localhome/patrixlinux # su postgres
postgres(at)patrix:/localhome/patrixlinux> cd
postgres(at)patrix:~> psql kv
Welcome to psql 7.4.2, the PostgreSQL interactive
terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

kv=#
"

So now, the user "postgres" still has no password.
How can I create the password for the user with psql?

How can I create an account and its password with sql
query from psql? And how can I define that they have
the privilege to work with a database and some
tables....?

Thank you very much in advance.

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