New pen pal in Japan? What to say?

You can start it just like you start communicating other people in your country. But just say which country and which town you live now to introduce yourself and your life.

Pictures, pictures, pictures. Unless your girl is a very unusual Japanese girl, her English won't be great. So, share pictures of your family and pets and home, with very simple subtitles. I'm talking subject, verb (present tense), object. "I like XXX" should go over well. "Do you like XXX?" may be almost too difficult . . . .

Ask about their interests - how do they spend their time - if they've ever been abroad before, what music and movies they like, their future ambitions, etc., etc.

You could start by introducing yourself, telling her about where you live, what your interests are, what you do in your spare time.