First, you have to find out the date that is six month older than now, to achieve this, use INTERVAL in PostgreSQL like this:
CURRENT_DATE - INTERVAL '6 months'
Now, you can retrieve all records that is older than this six-month-old date. i.e.
SELECT * from tablename where datefield > CURRENT_DATE - INTERVAL '6 months'
You should familiarize yourself with PostgreSQL: Documentation: 8.2: Date/Time Functions and Operators