Usually staining is not a separate step on outdoor furniture. You would use a penetrating sealer, which can be tinted to the color of your choice. There are also different transparancies of penetrating sealers available, the darker they are, the longer they will last.
Armitage is right about polyurethane and varnish being unsuitable. Those products are for indoors. They are "film build" products like paint is, but since they are clear they are destroyed by the sun. Paint could be used if you want, but then chipping and wear is a concern, and many people like the look of wood grain on their deck or furniture.