Question

draw a line segment ab and perpendicular bisector to it.

Answer 1

Draw a line Segment AB using a scale.
Then take a compass.  Adjust it to a length more than half of AB.  Take A as the center.    Then draw a circular arc on both sides of the line cutting the line.  With the same radius draw another arc with the center as B.

Join the two points of intersection of circular arcs.  This is the perpendicular bisector of AB.

See diagram.

Answer 2

Draw a line Segment AB
Now Take a compass.
Take length more than half of AB.
Taking A as the centre draw an arc on both sides.
 
 Taking B as the centre and  the same radius draw another arc.

Join the two points of intersection of circular arcs. 
This is the perpendicular bisector of AB.

See in the attachment . The perpendicular of AB.