How to trigger code execution?

How do I “trigger” vba code execution? Here is the problem and my sample code:

Sub test()
Range("A1").Select
If ActiveCell = 1 Then
ActiveCell.Offset(0, 1) = 2
End If
End Sub

I know that I can go to Run Macro, select macro I created and it will run, but it is not good enough for my situation.

I would like code to execute as soon as 1 is entered in A1 (as in above example.)

I would appreciate any help, suggestions.

alwaysgo,

Please read and follow forum rules below. Your code as per wrap for you this time.

Try the below code. Right click the sheet tab > select view code > then paste in the below

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address(0, 0) = "A1" Then
    With Target
        If .Value = 1 Then
               .Offset(0, 1) = 2
        End If
    End With
End If

End Sub

VBA Noob