How do I say my weight? In which SI units? I see people are using Kg as a unit of denoting weight from my childhood. Answer this question "What's your weight?".
Weight is a force and is expressed in Newton (N). Mass is expressed in kilogram (kg).
However, in informal (non-scientific) language, people often express weight in kg, although this is not correct strictly speaking.
The relation between the two is $F=mg$, with $F$ the weight (N), $m$ the mass (kg) and $g$ earth's gravity constant.
See also here.
As this is an Engineering stack we use the prescribed units, mass is in kg and weight is in Newtons.
This is a common misconception by the masses (great unwashed...) and it is also covered in this question : Force Required to Lift a WEIGHT of 1Kg
It is also obvious in various phrases such as "I am gong to boil the kettle" which is assumed to mean boil the water that is inside the kettle and not put the kettle in a big cooking pot and boil it...