Try turning the headlights on, and then have someone watching the lights while you try to start the van. If the headlights do not work at all or are weak when you turn them on; then the battery is probably discharged or worn out. If that happens, have the battery recharged and then tested with a battery load tester (just using a hydrometer or voltmeter cannot adequately test a battery). After the battery is recharged and the load test shows it has plenty of reserve capacity; or if the battery has been replaced with a good one; try the headlight test while running the starter again.
Generally, when there is a tick noise during that test; if the lights stay bright when you try to start, then the problem is in the starter, starter relay, or starter solenoid.
But if the lights are bright when you turn them on, and they dim when you run the starter; the battery is weak, or the battery cables need their connections cleaned and tightened. The battery will also become weak if the car's alternator is bad, and does not charge the battery when the motor runs. So the alternator should be tested if this is a possibility.
A starter with a shorted armature, or an engine that has seized up, will also make the lights dim; but those are very rare problems.