If you mean "a text file" by "notepad", then I think what you need to do is open a StreamWriter and output to that instead of the Console:
using (StreamWriter sw = new StreamWriter(@"D:\Temp\MyData.txt"))
{
switch (selectedOption)
{
case "1":
sw.WriteLine("\nSolution\n\n {0} + {1} = {2}", firstnumber, secondnumber, firstnumber + secondnumber);
break;
case "2":
sw.WriteLine("\nSolution\n\n {0} - {1} = {2}", firstnumber, secondnumber, firstnumber - secondnumber);
break;
case "3":
sw.WriteLine("\nSolution\n\n {0} * {1} = {2}", firstnumber, secondnumber, firstnumber * secondnumber);
break;
case "4":
sw.WriteLine("\nSolution\n\n {0} / {1} = {2}", firstnumber, secondnumber, firstnumber / secondnumber);
break;
}
}
(I took the liberty of fixing the output as well!)