pgaccess and postgresql

Hello,

Recently looked for a database system and was hinted towards postgresql in
combination with pgaccess.

I installed the necessary rpms on my redhat 7.0 and got the most recent
pgaccess.

Now I want to start playing around. I open pgaccess and try to create a new
database by using the menu: Database-New. I type in a name and upon
"create", pgaccess produces the following error:
>TCL error executing pg_exec create database xyz
>is not a valid postgresql connection.

Obviously I have neither experiences with postgresql nor with pgaccess. I
read in the pg documentation, that one is supposed to create a
postgresql-master account and start postgresql before using it. How do I
create a master account when installing via rpm?
How can I overcome the error message:

>postgres does not know where to find the database system data.  You >must
>specify the directory that contains the database system either >by
>specifying the -D invocation option or by setting the PGDATA >environment
>variable.

that shows upon calling "postgres" from bash?

Please help a clueless newby!

Thanks, Johannes
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