My knowledge here is limited to Nationalist China. Generalissimo Chiang Kai-shek learned about the attack on Pearl Harbor at around 1:00 A.M. on December 8. He proceeded to send a message of support (translated by his English-speaking wife Soong Meiling) to President Roosevelt: "To our new common battle we offer all we are and all we have, to stand with you until the Pacific and the world are free from the curse of brute force and endless perfidy."

At 8:00 A.M., Chiang announced his intentions before the Central Standing Committee. He wanted to propose that China, the United States, the United Kingdom, and the Soviet Union jointly declare war on the Axis powers and agree not to negotiate separate peaces. He also hoped for Soviet intervention against Japan. After the meeting, he read aloud the reply to his earlier message, in which Roosevelt said: "I take great pride in my country's association with you and the great nation which you lead."

China formally declared war on Japan, Germany, and Italy on December 9, accusing the three powers of conspiring "to carry out their common program of world conquest and domination." That same day, Chiang ordered his armies and guerrillas to step up offensive activities against the Japanese, ostensibly "to contain the enemy and prevent him from throwing his full force into the Pacific Theatre" (as the official histories explain), though more probably, I think, to reassure Roosevelt that China was committed to the Allied cause. Chinese forces advanced in the direction of Hong Kong to alleviate pressure on its British and Commonwealth defenders (the city fell anyway on December 25), while the Japanese attempted to divert Chinese attention by launching a third attack on Changsha. The Japanese were driven back by mid-January in the first Allied victory of the Asia-Pacific War.

Chiang also sent three crack divisions to Yunnan (along with most of his heavy artillery as well as the Flying Tigers) in preparation to reinforce the British in Burma. Although the British were initially unwilling to accept Chinese aid, that quickly changed as the Japanese pushed further into the colony. Of course, the First Burma Campaign turned out to be quite a fiasco for the Allies (though the New 38th Division under Gen. Sun Liren managed to rescue the British First Burma Division trapped at Yenangyaung in April 1942), but that's beyond the scope of your question.

I hope this helps! :)