Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I would say to get a wiring diagram either from mitchell1 or alldata and test for start signal from ign switch to starter solinoid if you have signal from switch at solinoid jump battery + cable to starter cable and see if it cranks if it does the its the solinoid. If you have signal at ign switch but not at solinoid then trace wire for a break in the wiring and probe it to see when you get signal. If you find it repair or replace wiring going back to solinoid.
If thats all good then i would look at the neutral safety switch which could be mounted on trans or in center console or on brake pedal/steering column. (Don't remember off top of my head) at this point i would check all fuses before checking switch to see if one is blown for that circuit that has a neutral switch on it.
