If you have any information about people affected by the disaster, you can enter it into the Google application. (You should especially use it if you know people are safe! It gives loved ones peace of mind.) If you have friends and family in Japan, you can also search the database and see if they are okay.
If you are in Japan, you can help with the mapping tool Ushahidi.
If you see people trapped or in need of medical attention, you can text the location, and it will appear on the Ushahidi map. You can also text places to avoid, places you can get aid, and other helpful tips.