Moving from MySQL

Hi all, new to pgsql yadda yadda

I have a default RedHat install and run into the following problem
When trying to connect to postgres I get the following message..
[kevin@anthem kevin]$ postgres
postgres does not know where to find the database system data.  You must specify the directory that contains the
databasesystem either by specifying the -D invocation option or by setting the PGDATA environment variable. 

ok, so I su to the user postgres and try...

bash-2.05b$ postgres -D /var/lib/pgsql/data/
Lock file "/var/lib/pgsql/data//postmaster.pid" already exists.
Is another postmaster (pid 16943) running in "/var/lib/pgsql/data/"?

What seems to be the problem here?

Kind regards
Kevin
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Kevin Waterson
Port Macquarie, Australia