if the starter isnt turning at all then first check,
battery, wire going to the starter, wire connecting the starter to the wiring harness (which is what recognizes ur key turning the starter, if all of that is good then take a hammer and bang on the starter sylenoid, give it a good tap and see if it comes to life, this is how u know when the sylenoid is dying because itll randomly cut out and come to life with a good jolt. if that doesnt cut it then pull the starter and take it to any parts store and they can test it. but start simple, recheck ur battery and connections, make sure ur depressing the brake petal when ur trying to start it, if its an auto make sure its all the way in park, and even if u think it is jolt it into park (my gf has a car that sometimes wont recognize it until u slam the auto shifter up pretty hard) neways try that hope it works for u and if it doesnt then let me know, good luck