Which day should i start counting the days for my next menstruation.i'm still don't know how to calculate it and how other girls know it so exactly.

Start of one to next: If i understand your question, by convention, a menstrual cycle is defined from the first day of menstrual bleeding as day one. The next period will start some 23-35 days (28 day average) after the start of the first. At that point, you start again with 'day one" of the next cycle. Saying they calculate suggests that they can predict their next cycle, but only if they are very regular.