Read about tsunamis in a reputable encyclopedia, such as Encyclopedia Britannica. You can find encyclopedias online or at your local library, and they may contain information about past tsunamis, particularly those that caused considerable damage. A basic internet search of the word "tsunami" will also give you a good starting point, but make sure information is cited before assuming that it is accurate.
Utilize libraries. Both public libraries and university libraries offer books and periodicals that can help you learn more about past tsunamis. Some periodicals that specialize on tsunamis are The Journal of Tsunami Society International and the Journal of Earthquake and Tsunami.
Access the article database of a university. These databases can be partially restricted to students and alumni, so try to access the university's database that you currently attend or that you graduated from first. From the database, you can search for articles from more general science journals and magazines that deal with past tsunamis. Research that is based out of regions that experience tsunamis, such as Japan, may offer more first-hand data than regions that don't ever experience tsunamis.
Check out resources available through the Tsunami Research Center. For example, this organization has articles posted about the 2011 tsunamis in Japan.
Read about past tsunamis on the "Center for Tsunamis Research" page on the National Oceanic and Atmospheric Administration's website. This organization posts information about research that has been done on past tsunamis.