Metric rulers are the easiest to read. They deal with centimeters and millimeters only. You wont have to worry about fractions.

Look at the following Metric Ruler.

The larger lines with numbers are centimeters, and the smallest lines are millimeters. Since millimeters are 1/10th of a centimeter, if you measure 3 marks after a centimeter, it is 1.3 centimeters long which is 13 millimeters (mm).

The measurement at the red line marked on the ruler is 123mm, because it is 3 of the smallest lines past the 12 centimeters mark.