Answer:
Basically, countries made a lot of alliances and when one country was attacked they basically had to help them out. When the archduke of Austria-Hungary got killed they wanted revenge by attacking Serbia. So Germany pushed Austria-Hungary to go fight them and they'll help them. So when Serbia was getting jumped, Russia had to go help them since they were allied. This set off a chain of alliances leading more and more countries into the war which eventually led to WW1.
