Not sure if your question is specific enough, but let's start with the code below and go from there:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim x As Integer
If Intersect(Target, Range("A1")) Is Nothing Then Exit Sub
x = x + 1
Sheets("Sheet2").Cells(x, "A") = "Y" & x
Loop Until x = Target
Implement code by right-clicking the drop down sheet's tab and selecting View code. Paste the code in the big white field.