12 Japanese professional baseball teams belong to Nippon Professional Baseball Organization. There are two professional leagues, the Central and Pacific League, consisting of 6 teams each. Baseball is one of the popular sports in Japan. Many baseball fans waited for the start of the season and the new season started yesterday. If you are interested in baseball, I recommend you to watch baseball games in a stadium or on TV. I think that watching sports is one of the best things to do during your stay in Japan.
You can search the schedules in the website of NPB (Nippon Professional Baseball).