security question

hello,

i'm very new to postgres and have a fundamental question.
how do i make a pg-db most secure?
i feel, that pg_user e.g. ist something very dangerous - isn't it? if 
someone hacks into a db, then he has lots of information at his/her 
fingertips. is this so?

what do i do to prevent my db from beeing hacked?

greetings
hans