Double up your query to return the last ID that was inserted:
Code:
    'Declare the object to store the last inserted ID
    Dim id As Integer = -1

    'Declare the connection object
    Dim con As OleDbConnection

    'Wrap code in Try/Catch
    Try
        'Set the connection object to a new instance
        'TODO: Change "My Connection String Here" with a valid connection string
        con = New OleDbConnection("My Connection String Here")

        'Create a new instance of the command object
        Using cmd As OleDbCommand = New OleDbCommand("Insert into Guias([Remetente],[Destinatário]) Values (@remetente, @destinatario); SELECT @@Identity AS [id];", con)
            'Parameterize the query
            With cmd.Parameters
                .AddWithValue("@remetente", ComboBox1.Text)
                .AddWithValue("@destinatario", ComboBox2.Text)
            End With

            'Open the connection
            con.Open()

            'Use ExecuteScalar to return a single value
            id = Convert.ToInt32(cmd.ExecuteScalar())

            'Close the connection
            con.Close()
        End Using
    Catch ex As Exception
        'Display the error
        Console.WriteLine(ex.Message)
    Finally
        'Check if the connection object was initialized
        If con IsNot Nothing Then
            If con.State = ConnectionState.Open Then
                'Close the connection if it was left open(exception thrown)
                con.Close()
            End If

            'Dispose of the connection object
            con.Dispose()
        End If
    End Try