Van will not start

It depends, if your saying it cranks good but will not start. Have a helper crank it while you check for spark at the spark plugs. If spark everywhere, use a gauge and check proper fuel pressure and fuel injector pulse.
Now, if your saying the starter motor will not crank, usually two voltage circuits at starter motor. One comes from battery, hot all the time, the other circuit goes hot with key in the crank position and is wired through the neutral switch, for auto transmission. A problem on that circuit could be ignition switch--neutral switch or wiring circuit. The starter motor grounds through engine block. Any testing at starter motor, make sure transmission is in park or neutral and functional parking brake is set. Make sure engine is not seized. Battery cables and connections are important, positive and negative, both ends.