You have to choose a decade of this century and investigate about it. You will have to write a timeline with the most interesting facts of the decade and make a poster with pictures and explanations.
You will also have to choose an important character and record a video with his or her biography. Here you have some useful web pages:
America´s Best History
History Timeline of the XXth century
Famous people of the XXth century
History of the XXth century
And here you have an interesting biography of John Lennon that you can use as an example to design your video: