Breaker in garage keeps tripping
Both your friends are right.  

Your electrician friend is right.  Wiring breakers are specifically sized for the load capability of the circuit.  If you put a 20A breaker on a 15A capable circuit, then congratulations, you've created a fire hazard!  (Circuit can now potentially overload carrying more amps than intended, overheat, and ignite.)

Your co-worker is right.  A breaker which trips repeatedly after replacing (wrong match or not) is not a problem with the breaker.  There is something wrong with the wiring or something connected to it.  Since you have nothing connected to it and it trips, that means your wiring has been damaged - possibly by a rodent of some kind.

Shut the breaker down for now and hire your first friend to troubleshoot it.  Electricity is not something to fiddle with if you don't know the rules well enough to avoid accidentally risking burning your garage down.  (Hope it isn't attached to the house!)