You can use Random.Next(int maxValue):
Return: A 32-bit signed integer greater than or equal to zero, and less than maxValue; that is, the range of return values ordinarily includes zero but not maxValue. However, if maxValue equals zero, maxValue is returned.
var r = new Random();
// print random integer >= 0 and < 100
Console.WriteLine(r.Next(100));
For this case however you could use Random.Next(int minValue, int maxValue), like this:
// print random integer >= 1 and < 101
Console.WriteLine(r.Next(1, 101);)
// or perhaps (if you have this specific case)
Console.WriteLine(r.Next(100) + 1);