Create a variable to store the sum and add to it whenever you find an odd number:
int sum = 0;
for (int X = 1; X <= 20; X++)
{
int y = rnd.Next(1, 10);
if (y % 2 == 1)
{
sum += y; // sum = sum + y;
}
Console.WriteLine("{0}", y);
}
Console.WriteLine("sum: {0}", sum);