Here it's often referred to as the Second Sino-Japanese war – which predates WWII –  the first having taken place in the mid 1890s. The Second Sino-Japanese war began in 1937 and ended with the end of WWII in 1945. The start of the war was actually just the final straw in a series of conflicts starting in 1931 between Japan and China, including the Japanese invasion of Manchuria as well as a sort of mini-war between Japan and China which took place in Shanghai in 1932.

If you want a good crash course, the Wikipedia article is actually fairly well written as an introduction, so that'd be a good place to start. I also highly recommend Shanghai 1937: Stalingrad on the Yangtze for a good overview of how things got started. It goes into some detail about the events in the early-to-mid 1930s as well.

China was quite active (if somewhat disorganised and misguided) in their efforts to combat the Japanese, and until 1941 had the support of the Germans as well, though this wasn't without some problems as well.

However it's also important to remember that China was also in the middle of their own civil war during the time of WWII and the War of Shanghai and the Second Sino-Japanese War. The civil war started around 1927 and lasted until the Nationalists were forced to flee to Taiwan and the Communists took over at the end of the 1940s. Internal conflict and the lack of a strong military means China wasn't exactly sending troops to the Western Front to help deal with the Nazis. However they were still very much involved in the conflict as part of the fighting with Japan.

We have users here with flair specifically for the Second Sino-Japanese war who could certainly provide some more detail. But in case they aren't able to get to your question, this should get you started.