こんばんは。
簡単ところでは、こんな感じかな?
Sub SaveMyBook()
Dim FileName As String
Dim myPath As String
'------------------------------------
'決まったフォルダがあれば、ここに入れる
myPath = ThisWorkbook.Path
'------------------------------------
FileName = ActiveSheet.Range("A1").Value
If FileName <> "" Then
If InStr(FileName, ".xls") = 0 Then
FileName = FileName & ".xls"
End If
End If
Application.DisplayAlerts = False
ThisWorkbook.SaveAs myPath & "\" & FileName
Application.DisplayAlerts = True
End Sub