Japan in WW1 was on the Entente's side, since they had the Anglo-Japanese pact with Britain.

They entered the war because of their foreign minister, an ardent anglophile and an imperialist Kato Takaaki, wanted to increase Japan's power and prestige( this isn't the only reason but it is the main one). The most notable battle was Tsingtao, a german port in China, which the Japanese soon took forcing the german navy (commanded by Von Spee) to scatter.

The Japanese didn't see much more action aftewards other than intercepting german and austro-hungarian ships since they didn't send men to the western front, although in East Asia by the time WW1 ended they had expanded their influence and Japan had effectively become more than just a regional power.

An interesting fact is that most of the people in power in Japan wanted an alliance with the Germans, because they saw their culture as closer to their own, but because of Kato Takaaki this didn't happen.

TL;DR The Japanese took away all of germany's colonies in East-Asia and engaged the Central Powers' navies, making a fairly big contribution to the war in Asia.

Sources: The First World War, Hew Strachan.