Won't state the obvious about wearing appropriately warm/dry clothing, hats/gloves/etc....you'll figure that stuff out pretty quick especially in the colder months.
If you're really pinching pennies and/or looking for a minimalist setup, you might want to take a look at "old" US/British webbing. Phenomenally flexible and quite cost effective. Packs down really well and doesn't take up huge amounts of space.
Or a "battle belt" where you can mount pouches/stuff right on the belt.
Don't forget about carrying water, any immediately necessary meds, a snack, kill rag. Really handy stuff, that you might not need for each game, are a pocket knife, compass, radio, pen/paper, flashlight, glowstick, tape, bit of rope....it's much easier to leave this stuff in your kit than to rummage around trying to find it all and stuff it into your pockets.
Get a good smart charger. It pays for itself by not killing your batteries. I don't know the options and price points now for these...the B6 charger is a great choice though. Buy good (G&P, Intellect, etc) NiMh batteries if you're not getting LiPo.
Get good quality BBs...BBBastard are great, Madbull are fine. Don't sweat getting a few more for a few bucks less...it's not a big deal in the grand scheme. Go for whichever is convenient since that's what you're probably going to buy in hurry when you next run out.
6mm plastic only. 0.20g for indoors...0.25g or heavier for outdoors (and indoors if they'll let you).
I don't know if your gun will come with a bb loader...but you'll want one.