How did WWI signal the end of colonialism?
Colonialism did not end after WWI. After that war, European powers (and others) still retained colonies all over the world. It was not until after WWII that many colonies started to gain their independence. However, it is still possible to argue that WWI led to decolonization.
One reason WWI (arguably) signaled the end of colonialism is that it reduced the respect that the colonized people had for the Europeans who dominated them. The Europeans had preached about the superiority of their civilization, thus legitimizing their rule in the eyes of many. The war showed, however, that European civilization was not really so superior. It was technologically superior, but it was not able to prevent a horrific war. This made colonized people somewhat less willing to believe that the Europeans deserved to control them.
Another reason WWI signaled the end of colonialism was that some colonies were angered when the colonial powers broke promises made to them. The colonial powers needed help from their colonies to fight the war. They used colonials as menial laborers in support of their armies and they used them as soldiers. They promised (or at least implied) that the colonies would be given independence or at least more freedom in return for their help. When this did not happen after the war, the colonies became more disenchanted.
Finally, we can say that WWI started people in the West thinking that empires were a bad thing. After WWI, Woodrow Wilson (president of the United States) issued his 14 Points, one of which held that people should enjoy the right to self-determination. While this right was not extended to non-white people after the war, the idea was still present. It became harder to defend the practice of subjugating non-white people while arguing that white people should have self-determination.
All of these are possible reasons WWI would have signaled the end of colonialism. You may want to consult your text and/or notes to see if any other reasons have been presented to you in class.