why array start with 0th location

This would have been an easy decision for the C# designers because the basic syntax of the language is heavily influenced by C which uses zero-based indexing for arrays.

In C array elements are laid out one after the other in memory and are typically accessed using pointers.

If the first element always has a subscript of zero, then this means that there is no need to deduct the subscript of the first element when working out the offset into the array at which a particular element is stored or to store the starting subscript itself. This clearly improves the efficiency of the underlying implementation. 

In C#, arrays are stored and accessed in a similar fashion though the underlying use of pointers is hidden from the programmer except in the case of 'unsafe' code.

http://en.wikipedia.org/wiki/Zero-based_numbering  Hope this may be of some use!