All vehicles are subject to road tax and for 2013 this is due to be paid by May 20th. You should receive an invoice in the post. However, if you have not received the notification, you will need to go to the Nerja Tax Office (Patronato de Recaudacion) at Calle Axarquia 3 (a side street off calle San Miguel opposite the entrance to avenida Castilla Perez) and ask for the tax bill. You can then go to your bank and pay it.