The choice of weight depends on your level and the muscles being worked.
There's a correct weight for every level. You should be feeling it when you finish your reps.
The lower body muscles (thighs, glutes, etc.) are larger than your upper body muscles (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)