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, too. Most of the functions are the same if only named differently.