Help with database

Hello,

I have impoerted data to postgresql database.

so i have few tables

geometry_columns
planet_osm_line
planet_osm_point
planet_osm_roads
planet_osm_polygon
spatial_ref_sys

I would like to select some usefull data.
For example nearest city from specified point.

Does anybody have information how to work with data selection?

Thank you
Pranas

http://wiki.postgresql.org/images/c/c5/PostGISPGUK.pdf

Or do you want OSM specific queries?