You can use function, like this:
Function FirstEmptyRow(wsh As Worksheet, Optional sColName As String = "A") As Long
FirstEmptyRow = wsh.Range(sColName & wsh.Rows.Count).End(xlUp).Row + 1
End Sub
Usage:
Dim lFirstEmptyRow AS Long
lFirstEmptyRow = FirstEmptyRow(ThisWorkbook.Worksheets("Sheet1"))
lFirstEmptyRow = FirstEmptyRow(ThisWorkbook.Worksheets(2), "B")
lFirstEmptyRow = FirstEmptyRow(Activesheet, "C")