Nejak takhle:
Sub ZapsatDoPrvniVolne()
Dim PoslRadek As Long, VolnaBunka As Range
PoslRadek = Worksheets("list1").Cells(Rows.Count, "A").End(xlUp).Row
Set VolnaBunka = Worksheets("list1").Range("a1").Offset(PoslRadek, 0)
VolnaBunka.Value = "neco"
End Sub