I need to study for a essay on tuesday and its on the 1940s but I dont exactly what. So any suggestions on how to start studing.

Three words for you: World. War. 2.

Also, look at the run-up to the Korean War (the ever-growing tensions between East - the Soviet Union - and West - the United States). Look at economic trends of the time, social trends of the time, the effect of the war on both of those things...

Really, without a more specific prompt, there's not much else you can do. Just look at the World War II chapter and the chapter about the aftermath (36 and 37 if I recall properly).