The first thing you need to look into is the pg_stat_* tables. There are a number of configuration parameters in postgresql.conf you need to change to enable them globally, including these (from my postgresql.conf file):
#stats_start_collector = true
#stats_reset_on_server_start = true
stats_command_string = true
#stats_row_level = false
#stats_block_level = false
This doc page will give you more information on the statistics views and functions.
