This can be one of a few things. Either a bad starter, ignition switch, or wiring problem.
First, check to make sure your battery is fully charged and you battery cables are tight and free of corrosion. If they are loose or corroded, tighten/clean them before any further diagnosis.
Next, lets check the starter;
While someone is holding the key in the "crank" position tap on the starter with a hammer and if the engine begins to turn over, you have a bad starter. Another way to check is with a test light. With the test light, you should have power at the single (thin) wire going to the starter with the key in the "crank" position. If so, you have a bad starter, if not, you have another problem.
You most likely have a bad starter, they go bad ALL THE TIME on these vans.
An easy way to check the neutral safety switch;
While holding the key in the "crank" position, wiggle the shifter around in park and in neutral. If it starts, you have a bad neutral safety switch and it will need to be replaced.
You can also try to check the ignition switch by wiggling the key around in it to see if that causes it to start, but you most likely have a bad starter.
I hope this helps! Let me know if you have any more questions!