http://www.livemocha.com/My brother is studying several languages for fun. He recommends this site for any language. My dad recommends for any language to look at Skypping with those people who speak that language. Surely there's some Japanese that want an American "penpal" over Skyppe. Also, can you host a foreign exchange student where you live?
Thirteen is old enough to read college English textbooks for fun. But be careful about getting college credit early. I read online that if you're not careful you'll mess up your scholarship by getting ahead. Look it up because I'm not sure. If it is just be careful to call it eighth grade work. It could just be an Internet rumor.
