Your question reeks of homework. But here's the answer to the first question.
public void printGradeInfo(int[] grades)
{
double min = max = averageSum = 0;
for (int i = 0; i < grades.length; i++)
{if (min > grades[i])
{min = grades[i]
}
if (max < grades[i])
{max = grades[i]
}
averageSum += grades[i]
}
Console.WriteLine(min);
Console.WriteLine(max);
Console.WriteLine(averageSum/grades.length);
}