How do I fix my van that will not start?

The most likely reason would be bad contacts on the starter relay or starter solenoid. You aren't getting enough amps to the starter.

https://shop.advanceautoparts.com/find/dodge-grand-caravan-starter-solenoid

have you checked the ignition switch thats on the steering column shaft, when you turn the key it moves a rod that moves positions on the switch.

Could be either one but the coil is the one that creates the spark but don't rule out wiring. I would have all 3 checked out

jumping across the solenoid will spin the starter but will not engage the pinion in the ring gear. Have a load test done on the battery as I suspect that there is a bad cell and you are not getting a full charge. If the battery is faulty then there is always enough power to turn on dash lights but nothing left to run the high current solenoid and starter.

ill answer for NO ELECTRICAL POWER :
sure and is easy with any DMM meter, got one? any $20 DMM.
my guess;1
engine will not crank
dash lamps out.
head lights out. dead.
post systems of electrical system , all parts that work and those that dont work, and problem is easy.
cleaned battery terminals.
tried borrowed battery that reads, 12.6vdc ? (charged is 12.6)
2012 that is in warranty car.... ?
my guess on 2012
jumping a car, that is battery jump starting.
a battery can short, and no jump start on earth will start shorted battery in car. but does lay down some big sparks, or burns up cheap walmart cables.
if the car will not crank, say just that, my starter motor will not crank
jump started from live running other car.
if true then make sure headlights work , if they dont work fix that first.
HL must always work, even keys in pocket.
best is to put a fully charged battery in the car, jump starting can cause
surges that can damage sensitive electronic$$$, do jumps as a desperate measure,
with a charged battery,
1: Head lights work, no fix that , use voltmeter
2: cranks robustly.
no , wiggle shifter lever, (on A/T cars the park switch blocks cranking) On M/T the clutch blocks cranking.
with any dMM
measure battery voltage before crank and during.
12.6v good. reseted.
cranking.
0 to 10v bad (weak battery or shorted starter motor)
11 and above good. and must crank, if not , starter is bad.
to prove starter is bad, check voltage at starter solenoid coil wire lug.
it must be at 11v cranking , below 8v it chatters, and fails.
if at 0v cranking this lug that means the Key start line is failing.
A/T switch not closes, or bad ignition switch.

Put key in the ignition,take it out of park and into neutral,wait few seconds,put it back into park and should start up.

it sounds like you may have a bad starter solenoid and the extra voltage from the jumper vehicle is enough to complete the circuit through the solenoid to allow the starter to turn over.sometimes the solenoid contacts get very dirty and can`t complete the circuit on its own without a jump.i think these have a bosch starter system on them and you can replace the contacts only in the solenoid.

Sounds like the starter itself has gone bad. If you can access it easily you can try to jump across the solenoid. Use a jumper wire from the hot lead to the exciter lead. If your not comfortable doing that DONT. Im pretty certain your starter is bad.

I am not familiar with the machanics of this car, but just one click is either a starter or a starter solenoid. As this may be a Ford derivative, solenoid is probably remote, between the battery and the starter. Remote type, usually mounted on inside fenser wall, have large gauge wire from battery to one side and a large gauge wire from the other, with small wire(s) for starting cicuit - check connections for small wires first with cleaning and reattaching. Solenoid itself can go bad, can test by jumping across from large post to large post (CAREFULLY); if car starts, then solenoid is bad.

probably dirty connection on the battery.The headlights should come on bright, then dim only slightly when the starter engages.
Remove the battery cables and clean all corrosion off the terminals and connectors. The lead must be bright and shiny to make a good connection. Once you have checked and fixed the battery connections, and still no crank, find the starter relay and tap it firmly ,then try again. Still no crank, manually jump across the relay to 'force' current to the starter solenoid. Still no crank--replace the solenoid