Why boys are usually taller than girls ?

Boys and girls are actually about same height on average - until they reach the age of 12 or 13. After that age, girls' growth starts to level off. Boy continue growing until age 17 or 18.
Boys and girls grow about two inches per year until they reach puberty. Then they each have a growth spurt that lasts a year or two. During the growth spurts, boys grow a little faster than girls. Girls reach puberty earlier than boys. The differences in when they reach puberty , how fast they grow during it, and how long puberty lasts are what make boys average about five inches taller than girls by the time they stop growing.

To estimate how tall you will be when you are fully grown, you can use the knowledge that men are on five inches taller than women. If you are a girl, subtract 5 inches from your father's height. Then add tour mother's height and then divide by 2. This will give you the average height your parents would have if they were both women.
If you are a boy, add 5 inches to your mother's height then add your fathers height and divide by two. This gives you average height your parents would be if they were both men.
The problem with these estimates is that they will only let you guess your height within about 4 inches. That is a lot of variation. On average, siblings of the same sex will be within 4 inches of one another.
There is another way to estimate your adult height that indicates just how unreliable height estimation is. If you know how tall you were when you were 2 years old, you can double that to estimate your adult height. Notice that this method does not ask whether you are boy or a girl. Remember that boys and girls grow at about same rate until puberty and that there is about five inch difference on average between men and women's height. So this estimate may easily be up to five inches off.