Roman numerals are made by combining seven symbols into specific groupings. I represents the ones, V represents the fives, X represents the tens, L represents the fifties, C represents the one hundreds, D represents the five hundreds, and M represents the one thousands.
Numbers one through 10 in Roman numerals are expressed as follows: I, II, III, IV, V, VI, VII, VIII, IX, X. One, two and three are simply one two and three lines. Four is represented essentially as one less than five, IV. V is five, six is five plus one, or VI, and so on. Nine is 10 minus one, or IX. This pattern continues all the way to 3,999. To break down a larger number, use the current year - 2015. Two thousand is expressed with two of the thousand symbol, MM. Fifteen is ten plus five or XV. So the year 2015 in Roman numerals is MMXV.
Smaller number symbols almost always go to the right of the larger number, unless doing so would put four symbols in a row, in which case the smaller number goes to the left and is subtracted from the larger number. This means nine is shown as one less than 10, or IX, rather than five plus four, or VIIII. This is called the subtractive rule, and it is explained in detail at Roman-Numerals.org. The smaller valued symbol can only be on the left when it is 1/10th of the larger number. This means 99 cannot be shown as one less than 100, or IC. It is broken down further into 90, which is 10 less than 100, or XC, and nine, IX. Ninety-nine is XCIX.