You don't give any information about your tables. Assuming the point isn't in a table and venues has fields id, radius, geom...

    select
       venue.id
    from
       venue
    where
       st_intersects( 'Point( 0 0 )'::geometry, st_buffer(venu.geom, venue.radius) )


I recommend playing with SpatiaLite to learn about the basic geometry operations. It's easier to set up and get going than PostGIS. They have [a nice tutorial][1], too. Most of the functions are the same if only named differently.


  [1]: http://www.gaia-gis.it/spatialite/spatialite-tutorial-2.3.1.html