Start with checking the transmission fluid level and since the transmission coolant system is cooled by the radiator I'd start with checking to make sure the rad coolant isn't worn out or low or mixed wrong.Use a anti-freeze tester and check the quality of the coolant , plus be sure to check for any leaks in the rad or coolant system or transmission lines etc.
If the vehicle has over 75,000 or on it I'd be sure to drain the rad and coolant system and drain the transmission fluid , plus replace the transmission filter on it,also change the gas filter especially if you've not done any of these things before and it has several miles on.
Hope that helps and best of luck.By the way it might not be a bad idea to pick up a service manual for that vehicle.