automatic transmission not shifting

First thing: check your fluid if you haven't already. Lack of fluid or worn out fluid can cause automatic transmissions to fail to shift properly.

I don't know what the specific steps are for a Dodge Caravan but there should be instructions in the owners' manual. Usually it involves something like the following:

1. Drive the car to fully warm up the transmission.
2. With your foot on the brake, shift through each gear range manually, holding for a few seconds. Then shift back into park and remove your foot from the brake.
3. With the engine running, open the hood and pull out the transmission fluid dipstick, wipe it clean, stick it all the way back in, and pull out again to check level.

The above steps are something like what's required on some Chevy trucks. Some vehicles require checking while the engine is off but most, I think, require a hot check while the engine is running. I could be wrong. Check your manual.

If the fluid level is OK, sniff it (don't burn your nose!) and if it smells "burnt" then it could need to be changed. Auto transmission fluid changes are usually kind of messy so you might want to have this done somewhere.

If the level is low then you need to either look up the type of fluid it needs and add some (check the manual again) or take it to a mechanic to get it topped up.

Good luck!

Getting a check for codes is a good idea too but a fluid level check is a quick no-brainer test to rule out a really simple problem first.