Hey there!
You need not necessarily start preparation from 8th std. You can focus on 8th std now. Do well in ur current class. You can start preparation from 11th. You can either join in a coaching center or prepare yourself for the exam when u finish cls 10th. There are many foundation courses in 8th for neet . You can join in any of them as you wish!
Best wishes! 😊
-SWAPNA