Becoming a doctor in Japan

Can people from America who move straight to Japan after highschool become a doctor in Japan?

Are you asking if a U.S. high school diploma will qualify you to practice medicine in Japan? If so, the answer is no.

no im asking if I can go to medical school and become a doctor in Japan

If they study the local language, pass the entrance exam to a medical school (not easy even among the local students), go through the six years of study of medical school (with associated tuition and expenses), etc., like other Japanese students, yes.

Can you speak, read and write fluent Japanese? That will be the first hurdle.

the second hurdle is that you need to have school records by which you can enter medical school in USA (although the system is different that in US medical school is graduate school, but, in Japan, medical school is undergraduate.)
so my opinion is that you can become a medical doctor in USA easier than that in Japan, because you don't need to learn the language.
I can say that if you have a school record that you are No.1 out of thousand of the same age (i.e. you are the top in your school), you may be qualified in terms of school record. but, your language level will be the lowest among the competitors.