A ruler has lines on both sides, one set of lines that are really close together (this is the metric system used throughout the world) and one set of lines that aren't as close together (this is what people normally use in the U.S.)
On the metric side, where the lines are close together, the lines come in three sizes: small, medium, and long. Start at 0 and count each line until you reach the medium line. You should have counted four of the smallest lines and the medium line as the 5th line. Keep counting until you reach the longest line with the number 1 at the end. You should have counted to nine with the longest line (the one with the number at the end) as the 10th line. What you've just counted is 10 millimeters within 1 centimeter. That means that medium line is the same as 5 millimeters, which is also the same as 0.5 centimeters. Counting from 0 to 2 of the smallest lines would be 2 millimeters, which is the same as 0.2 centimeters.
On the U.S. side, things are a bit different. The lines are a littler further apart, there are four different lengths, and there aren't as many small lines between the long lines with numbers at the end. That's because now we're counting in inches, and this side of the ruler is divided up into inches, half inches, quarter inches, and eighth inches. In this case, you would read it like this:
-- 1/8 inch
---- 2/8 inch, also called 1/4 inch
-- 3/8 inch
------ 4/8 inch, also called 2/4 inch, also called 1/2 inch
-- 5/8 inch
---- 6/8 inch, also called 3/4 inch
-- 7/8 inch
-------- 1 inch.
Hopefully the formatting doesn't change when I post this, and I hope this helps.