Macro: Select cell A of selected row

Hello,

Currently working on a project for university and still learning how to use macro.
Got a problem which im sure is an easy solution for all you experts out there

As shown in the picture;
http://img300.imageshack.us/img300/1205/macrohelp.jpg

When the Sap number is entered (from A19 and onwards), it copies the value to cell B10. The general code is used below.

Private Sub Worksheet_SelectionChange(ByVal Target As Excel.Range)
If Target.Value <> "" Then
ActiveSheet.Range("B10") = Target.Value
End If
End Sub

I want to make it that when any cell in the row is chosen, it will lookup the corresponding A column value and copy into B10.

Hopefully someone can help me with this

Thanks so much!
- Patrick