How you got to be a 'lefty'
Well scientists are not sure about why around 85-90% of people are right handed and the others turn out to be left handed.
They have discovered that:
Left handedness often runs in families. So it seems that your genes have something to do with it. Someone else in your family might be left handed too. However, many left handed kids have right handed parents and brothers and sisters. How genes work is often rather unclear.
More boys than girls are left handed.
Some people who have injured their right hand have to learn to become left handed.
Some people use their left hand for most things but can use their right hand too. This is called being ambidextrous (say am-bi-dex-truss) but only about 1% of all people are truly able to do everything really easily and well with both hands.