Hang a weight, a stone at a rope and it always points straight down when it hangs motionless. The stretched wire has a perpendicular line.

schietloodWith string (or compass) you can construct a right angle.
Draw a straight line (a stretched string) and place a point c. From this center draw a circle. With the same radius, draw from each intersection of the ring with the line again a circle. The compound of the intersection of two circles is perpendicular to the first line.
That 90 degree angle can further be divided into 2 x 45 or 3x 30 or 90 x 1 degree.