Here are a range of resources which might help you understand the early Cold War period.A good introduction is School History’s page on the Origins of the Cold War.
You can gain a similar overview from the History Learning site and its comprehensive index page that covers almost every event in the Cold War.
The BBC has a good page examining the role of Truman in the early Cold War, while its Bitesize section is perfect for revision. Give the podcast on this page a listen to as well.
It is also worth taking the time to listen to Winston Churchill's famous "Iron Curtain" speech made in Fulton, Missouri in the United States in March 1946:
