Causes of WWI: The immediate cause of WWI was the assassination of Archduke Franz Ferdinand by a Serbian terrorist group called the Black hand. Other things that allowed this to escalate to WWI were nationalism imperialism, and alliances between countries.
Fighting of WWI: Most of WWI was fought defensively, in trenches, making it hard for either side to attack. Because if they came out of the trenches they could be hit by the opposing side. Many people were injured, if not killed.
Post war: The allies won WWI and created the treaty of Versailles stating that Germany was at fault in the war and needed to pay for all the resulting damages. Germany resents this causing tension that leads to WWII.