Use rulers and compasses to draw a line which is perpendicular to line AB at point C

Draw a line AB

Take a point C on AB

Fix the tip of compass at C and with a fix radius, cut AB on both sides of C

Mark those points D and E

Fix the tip of compass on D, and with the same radius, draw an arc above C

Do the same with E

Those two arcs will intersect at a point, mark the point F

Make a line passing through both C and F

Line CF is perpendicular to AB