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) )
This goes through the venues table and
- reads the point geometry stored for the venue in venues.geom,
- draws a circle around that point with the radius stored in venues.radius,
- and tests whether the POINT(0 0) intersects that circle.
It returns the venue.id if they intersect.
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.