How did president truman plan to prevent communism from spreading to new areas of the world? what was his plan called?

Truman's plan was for the United States to provide aid in places where communism became a threat.  This became known as the Truman Doctrine.

In response to a civil war in Greece in which communism was threatening to take hold of the country, President Truman announced what became known as the Truman Doctrine of intervention to help those threatened by communism.  In a March, 1947, in an address to Congress, Truman said:  "I believe it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures. I believe that we must assist free peoples to work out their own destinies in their own way. I believe that our help should be primarily through economic and financial aid which is essential to economic stability and orderly political processes."