Thermostat busted? Water pump not pumping? Temp sensor for the fan not working? Coolant system need to be flushed? Radiator clogged with insect carcasses? Check these in reverse order (bug problem is easiest, thermostat most involved).
If the radiator is dirty, go to a do-it-yourself wash bay and use the high pressure sprayer on it. Open the hood and spray into the rad from the back (going from the front only pushes the gunk further into the cooling fins).
If that isn't it, go somewhere where you can get the cooling system flushed (preferably power-flushed). That should clean out any sediment that has built up in both the rad and the engine block. This is actually a pretty simple procedure, but it is messy, and has a high potential for dumping coolant all over the ground. Shops have equipment to catch any and all coolant, and can send it off to be recycled. Yes, you will have to buy more coolant, but then, an engine is so much more expensive (and dodge minivan engines aren't exactly paragons of durability).
Your local library should have a repair manual you can use to look up how to check the sensor for the electric cooling fan (or sneak the wrapper off the appropriate manual at Barnes and Noble
). The sensor itself should be pretty cheap. Alternatively, if you have a mechanical fan (engine-turned), get the motor good and hot, then shut it off and see if the mechanical clutch on the fan is engaged. If you can turn the fan relatively easily, then you need a new fan clutch.
Next, wait for the radiator to cool a bit, until you can comfortably touch the radiator cap, then open the radiator and start the engine. See if the coolant is flowing adequately. If not, your pump is fragged or the thermostat is stuck closed. Unless you are mechanically inclined, that is a job for your local mechanic.
If you do take on either of the last two jobs yourself, use a repair manual. With so much aluminum on today's engines, all it takes is one overtorqued bolt to strip an important thread and ruin your day, and maybe the engine block. Have Heli-Coils handy just in case, and know how to use them.