I cannot believe I'm writing a post about socks, but here goes. For PT, you'll want white crew socks. Bring a bunch. I changed out my sweat/dew/water soaked skivvies and socks after PT for hygiene's sake. The less markings the better, so you don't stand out. Crew length as you'll put your gogue pack (ziplock bag with notebook, pen, and gogue book) in there during InDoc. Sometimes you'll "demotivate them", i.e. roll them down. Only when you're told though. Also, I want to punch whoever came up with "demotivate" socks.
For NWUs, you'll be force to buy the NWU boot sock. It's actually a pretty good IMO.
For CandiO phase, get a few extra pairs of nicer black dress socks at the NEX when you are able to go there as a Candio. The issue ones are garbage. Don't get a bunch though, you need tan dress socks in PCola to put with the brown shoes you'll be wearing here.