When doing vehicle maintenance, it's important to know the difference between a defective motor starter and a breakdown else where in the electrical system. You may need electrical maintenance elsewhere if the motor starter is shown to be working properly. You could have a problem with the ignition switch circuit, the push button, the wiring or the solenoid.
Ruling out these systems one by one can reveal the problem. You need a volt meter to test for most of the problems that might rule out the motor starter. One of the most basic indicators of a problem is low voltage or no voltage to a critical area. Are you getting voltage to the solenoid when trying to start the engine? Does current flow to the solenoid but not to the main contacts? There are many things to explore in this area. Don't assume your problem lies with the motor starter--there may be a related issue which prevents the motor starter from doing its job. If you don't know how to use a volt meter, take your vehicle to a professional for evaluation.