You need to hook the worksheet's SelectionChange event.
Go into the VB editor (Alt+F11).
Double-Click the Sheet2 Object.
In the left drop-down box, select Worksheet. That should automatically add a SelectionChange event.
If not, use the right drop-down to select SelectionChange.
Then, in the code block, just write:
Sheet1.Range("A2").Value = Target.Value