Does the engine crank? And not start?
Or, does the engine crank and not start?
If it does not crank then check the battery and terminals for corrosion.
See if there is a bad ground. Test light on negative terminal and the light touching the engine block, then crank. If Test light bulb lights up then bad ground.
If not then check the starter relay. See youtube videos on how to test a (starter) relay
If engine cranks but does fire up then check for the basics
Fuel, spark, compression, timing, exhaust (blockages)