The choice of weight depends on your level and muscles you want to work.
The right weight is what corresponds to your level. You should be able to complete your reps but be tired at the end.
The lower body muscles (thighs, glutes, etc.) are larger than your upper body (shoulders, biceps, etc.)
This means you should choose heavier weights for doing squats or lunges (2 kg and up) and use half that weight for smaller upper body muscles (1 kg)