The Text property of a TextBox is a string - so adding them together doesn't work ("4" + "4" = "44")
So you need to convert the string to a number.
Lets assume you are using integers.
you can do
int number1, number2;
if (int.TryParse(txtFirstNumnebr.Text, out number1) && (int.TryParse(txtSecondNumber.Text, out number2))
{
int result = number1 + number2;
txtresult.Text = result.ToString();
}
int.TryParse attempts to convert teh string to an integer, returning false if it fails, or true if it works, and puts the result in the out parameter (number1 or number2)
I've shown an 'ADD' example - the same would need to be done for all the other operations too
Of course you could vastly improve on that by adding validation first, with error messages etc. - but at the level I am guessing you are at from your question, this should get you started.