The vocabulary of TOEFL is around 8,000, so even if junior high school English is good, the challenge is still great. So I suggest you start preparing now. Generally speaking, there are many ways to learn TOEFL. If you prepare according to your opinion, you can speak your English in a year. The TOEFL test time is before July and August of the second semester of junior high school at the latest, because in September, foreign countries generally begin to accept online application for schools, which can guarantee the application to a good school. Of course, it is not recommended to take the TOEFL test too early. The validity period is 2 years, which is too early, and it will be bad after the validity period.
Generally, the requirement for TOEFL in American undergraduate schools is 80+, but the requirement for junior high school students may be lower. You can go to various schools in official website to find out.