Maybe a bad starter, maybe not. You need to do a little detective work with a voltmeter to determine where the problem is at.
First off make sure the connections at the battery and engine are good (clean the terminals etc.)
All of these measurements need to be taken while someone cranks the starter for a few seconds: Measure the voltage from the + battery terminal to the + connection on the solenoid. Likewise, measure the voltage across the solenoid. Measure the voltage between the output lug on the solenoid and the + post on the starter. Finally measure the voltage between the - battery terminal and a good ground on the engine. If any of these measurements are above approximately one volt you've located a problem. If all of those measurements are good and you have solid, clean connections at the battery and engine then the likely culprit is the starter.
