There were several factors at play that led to the Allies' victory in World War I. In some ways, the Allies were simply more fortunate than the Central Powers, but they also made several strategic decisions that assured their victory. Mostly though, the Allies (with the exception of Russia) were simply able to outlast their opponents.
A simple but significant advantage that the Allies had was manpower. When the war started, they had about five times the population of the Central Powers and significantly more territory. As a result, they had more manpower and resources to draw from.
The British were able to successfully blockade German ports. With so many of their farmers at war, Germany was unable to grow enough food to feed its population. With their ports blockaded, they were unable to import food as well. As a result, much of Germany was on the brink of starvation after a few years of warfare. This led to unrest and poor morale on the home front. Although the Germans did attempt to prevent the Allies from importing food and supplies using their navy, particularly u-boats, they were ultimately no match for their adversaries.
The Central Powers also made the mistake of antagonizing the United States. Germany, in particular, made enemies of the Americans by attacking US ships and attempting to get Mexico to invade parts of the US. When the Americans entered the war in 1917, they brought fresh energy into a fight in which both sides were exhausted. With their economies collapsing and their reserves of soldiers nearly depleted, the Central Powers ultimately gave up the fight.