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.
Your lower-body muscles (thighs, buttocks, etc.) are larger than your upper-body muscles (shoulders, biceps, etc.)
You should therefore use heavier weights for squats and lunges (from 2kg) and a lighter load for your smaller upper-body muscles (1kg).