empty fields

hello,
I have 3 textbox in the save.vb and a button called save
I want to make sure the ppl dont leave empty fields
meaning if they only put name and last name but not
phone a windows will come and say you need to put
your phone number. how would I do that?

Public Class frmAdd
    Private Sub cmdSave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdSave.Click
        Dim FileNo As Short
        On Error Resume Next

        FileNo = FreeFile()
        FileOpen(FileNo, "C:\Accounts.ini", OpenMode.Append, , OpenShare.Shared)
        PrintLine(FileNo, txtFirst.Text & ";" & txtLast.Text & ";" & txtPhone.Text)
        FileClose(FileNo)

        frmMain.Enabled = True
        frmMain.Activate()
        Me.Close()
    End Sub
End Class