If you haven't already, jump start the van using jumper cables and another car. If it starts, it's a good sign the battery is weak. Just because lights come on when you turn the key doesn't mean the battery is fully charged.
If the battery is good, turn the key and
tap the starter with a hammer. If it starts, you'll either need to replace the solenoid contacts in the starter or replace the starter.
Using a volt meter check for 12vdc at the starter connection where the smaller diameter wire connects. If it's there when you turn the key to start, you likely have a bad starter.
Here's how you troubleshoot and change a starter:
http://www.autozone.com/az/cds/en_us...rInfoPages.htm