well, "kind of like a calculater" would imply,
a command button for each function (add,sub,div, mult).
Thus, in each click event of your buttons, call your Sub()
with the appropriate argument.
Private Sub cmdAdd_Click()
Call Calculate(1)
End Sub
Private Sub cmdSubtract_Click()
Call Calculate(2)
Ens Sub
Private Sub cmdMultpily_Click()
Call Calculate(3)
Ens Sub
Sub Calculate(intFunction As Integer)
Select Case intFunction
Case 1
Me.txtTotal = Me.txtFirst + Me.txtSecond
Case 2
Me.txtTotal = Me.txtFirst - Me.txtSecond
Case 3
Me.txtTotal = Me.txtFirst * Me.txtSecond
...a little primitive but, it addresses your question.