I'm not going to do the job of your teacher. I'm assuming you're in High School which means you have a state supplied textbook which should have information on this. I would suggest starting there. Otherwise, you can the basic information you seek from here:

http://www.firstworldwar.com/origins/causes.htm

Please don't take this link as the be all end all fountain of information on this topic, it's just a basic high school understanding of the topic and please try to pay attention to your teachers lectures next time. "He didn't teach us it" is the age old excuse of students who were too busy in the mountains with Shakira in their daydreams. Again, please read your textbook. It should have all the information you need.