Electric sander of some kind, I hope, for the sake of your arms and shoulders.
Depends on the method and what kind of stain.
Light stain, I'd say 120 grit with the random-orbit. Dark stain and varnish, straight-line sander to 150.
Casey

Thank you, Casey. I do have an orbital sander. But I'll be using dark stain for exteriors. I'm not familiar with a straight line sander. Does it go along the grain?

Just don't go overboard with the sanding, SYP has a good deal of variance in density between earlywood and latewood. It's easy to get a rippled texture with over-sanding. There was a thread here a few months back about a piece that had rotary cut pine and was over-sanded and stained dark. Looked terrible, IMO.

It's probably not the best sanding dust to breathe, either, hook up a vac to your sander.