The comment (or the answer) provided by Ashraff Ali Wahab tells you about the theoretical part of the algorithm which is great. You might have a look this page
Sorting Algorithms Codes in C#.NET[
^]. It will help you to start.
The answer given by Marcus is also good, as you could quickly search the google. Research with the problem is really good learning process. All the best in learning Mergesort:)