First thing first, do you even know what ALT is? How people ( Japanese and foreigner) perceive ALT as?
Have you heard of dancing monkey, tape recorder, low pay etc.
Next is why would you want to be an ALT, when you (a Japanese national) having just graduated from high school, could just continue your study at university, earn a teaching licence to become a proper English Teacher, if you really into teaching English.
Most ALT work for dispatch company "h", like Interact,AEON, Borderlink etc. This company get contract from Board of Education (BOE) to 'assist in teaching English' at school, where they will then hire mostly newly graduate foreigner, who have no whatsoever experience/interest in teaching. This 'teacher' have their work visa sponsor by the dispatch company, mostly on 1 year contract. Those 'teacher' will then be given simple basic training in 'teaching English Japan way' but most of it you will learn/figure out when you actually start 'teaching' at school.
They also hire resident status holder( spouse,PR etc.), going for Filipina housewives on lower salary.
Average ALT salary can be as low as 160,000 yen or upto 250,000 yen. JET ALT are the best paid 280,000 yen.
Since you study in Japan, you shoul have experience lesson with ALT when in school.
Most ALT will be assign to few school which they will go to on different days of the week. In kindergarten or elementary school, most of the time ALT will do the teaching on their own(which is basically illegal since ALT don7t have licence to conduct teaching), maybe lucky if the assigned a teacher that help you out. As for junior/senior high school, ALT normally follow whatever the Japanese Teacher of English (JTE) instruction.
So kindy and elementary, ALT will be 'dancing monkey' .. sing/chant/games or whatever to make English lesson 'fun'. JHS/SHS ALT mostly readout from textbook (rather than JTE play the CD), play games and 'communicate' with students.
Not to say ALT is bad, most people use 'teching English' as a door to enter Japan, maybe do 1-2 years work, networking and move on to other better job.
Ifve also been told from other people that some companies will train you for a fee and you will get a certificate or a license? I am not quite sure. But they said that once you get that you can be an ALT. That is just BS. Certificate/licence for what? Any tom dick harry with a degree can be an ALT. Unless they are talking about CELTA or TESL?
So again the question why you, a Japanese national want to wok as ALT, when you could be a licence English Teacher.
Read this.
https://blog.gaijinpot.com/japan101/teaching-english-in-japan/