I am divorced for 11 years. I have 2 children from my marriage. My elder son is getting 22 years old and graduating from college. My younger son is 14. By our agreement I have to pay child support for 2 kids up until each one of them reaches 22/graduate from college. However, there is nothing there that would say how much I have to pay when the first one turns 22. How do I go about it?

You file for a modification of support in the court that has jurisdiction (usually the court that issued the first CS order). It should be a fairly simple process.