its the starter. tap it with a hammer then try to start. if it starts you know for sure that it is the starter solenoid (in the starter assmbly, you usually replace it as a whole). happens on older cars.
well first check the connections on the starter, but if its the original starter, or near 10 years old, think of it as preventative maintenance replacing the starter.
the starter is on the flywheel/tranny side of the engine/driver side engine bay, as it turns the flywheel to start the engine. at the bottom, right under the oil filter (if you have changed your own oil before...you know the part that you cant miss when all the oil from the filter comes out?)