There are a lot of things to consider when painting or staining wood outdoors.
 
What's on there now? Oil or water-based paint or stain?

Does the wood need to be stripped or cleaned?

Is there any rotted wood that needs to be repaired/replaced?

Do any planks need to be repaired or replaced?

What kind of paint / stain? Opaque, semi-transparent, transparent?

What color?

What sheen?
 
Yep. It's more complicated than just coming picking a color and brushing it on!