Put this in a module for the sheet. Right-click the sheet tab, choose 'View Code' and copy/paste the code below into the white space.
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Count > 1 Then Exit Sub
If Not Intersect(Target, [B1]) Is Nothing Then
MsgBox "Cell B1 was changed" ' here you can call the macro you want to fire