Newbie's question: How can I connect to my postgresql-server?

Dear my friends....

I am still new in PostgreSQL. Usually I use MySQL on OpenSuSE.

I just have installed postgres with: "zypper install
postgresql-server". The installation looked OK.

As post-installation process, I've done:
"
sussy:~ # rcpostgresql status
Checking for PostgreSQL:                                             running
sussy:~ # su postgres
postgres@sussy:/root> createuser -D -p ricky
could not change directory to "/root"
Enter name of role to add: ricky
Shall the new role be a superuser? (y/n) y
createuser: could not connect to database postgres: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/tmp/.s.PGSQL.0"?
postgres@sussy:/root>
".

Why can I not connect to my Postgres?

Please tell me. Give me a first hint and this small step for me to
getting start to use postgres.

Thank you very much in advance.