Hey,
The syllabus of NEET and AIIMS include class 11th and 12th  Physics,Chemistry and Biology.
In class 8th,you should try to build concepts. Try to learn instead of mugging up. Develop analytical skills. And be sure of your calling. If you really want to be a doctor then only move further in this field. Talk to people who are already in this field.
Hope this helps.
Good luck.

Just read ncert and if you want you can study biology books of higher classes .

And mtg book has decent number of mcqs you can.practice that too.

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