understanding afr

I just finish the afr course and I still dont know how to choose the correct afr for specific engine

do i have to take another course to understand

You determine the ideal AFR through testing -- First tuning the engine to meet the target AFR, then by trying different AFRs to see which best meets your needs (such as maximum power, or minimum fuel consumption, or sometimes a reliability component such as reducing combustion temperatures).

As David said, different engines will respond differently, even some apparently very similar setups may differ in lambda, boost, timing, etc.

For a petrol/gasoline engine, around 10:1 may be a good start point and you can test from there - but I'd suggest getting used to using lambda instead of AFR, as the AFR changes for different fuels but the lambda is relatively independent of the fuel used.