How do you write zero in Roman numerals?

You don't. See Charles Seife's "Zero: The Biography of a Dangerous Idea" (New York, 2000), p. 55:

"In the year 525...all Westerners clung to the clunky Roman style of numbers, and there was no zero in that counting system."

The passage refers to Dionysius Exiguus' attempt to fix the date of Christ's birth.

The concept of zero was introduced to Europe in the middle ages via Muslim Spain.

Zero can't be written in Roman Numerals because it is introduced by Indians after roman period.The example for this is:-10 written in Roman Numerals->XI(X=10;I=1){10-1=9=XI}.

Strictly Roman numbers should be written without using the MCM for 1900 type of abbreviation if you are going to do maths. This makes addition and subtraction possible (but carving dates rather tedious). As it is a non-positional system (symbols have absolute values - C is 100 no matter where you put it), there are no spaces to fill (as there are in 1900. Addition is simply a matter of putting everything together, and then arranging into groups and changing to higher groupings when appropriate (bit like carrying)). Subtraction can be attempted first by cancellation of common factors, then by the reverse of carrying (sort of!). Multiplication has got to be multiple addition (yuk!) and division is best not even thought about....

In general, the number zero did not have its own Roman numeral, but the concept of zero as a number was known by medieval computists (responsible for calculating the date of Easter). They included zero (via the Latin word nullus meaning none) as one of nineteen epacts, or the age of the moon on March 22. The first three epacts were nullae, xi, and xxii (written in minuscule or lower case). The first known computist to use zero was Dionysius Exiguus in 525. Only one instance of a Roman numeral for zero is known. About 725, Bede or one of his colleagues used the letter N, the initial of nullae, in a table of epacts, all written in Roman numerals.

A notation for the value zero is quite distinct from the role of the digit zero in a positional notation system. The lack of a zero digit prevented Roman numerals from developing into a positional notation, and led to their gradual replacement by Hindu-Arabic numerals in the early second millennium. On the other hand, the lack of positional notation may have prevented the Romans from developing a "zero". Which affected which is not certain.