Top to bottom?
Bug spray. Put it on away from your goggles at the beginning of the day, re-apply as needed throughout the day. Head to toe. Be prepared to do a tick check when you get home if your area has ticks.
Head: Goggles you can wear for hours on end. You might be on the field for extended periods of time. If your goggles came with a chin strap, and your field's rentals have a chin strap, wear yours. It sets a good example for the renters. You may find you like it, too. Boonie hat is a solid suggestion. I run my visor, not just because it's nice to have a bit of shade, but it can also bounce/absorb shots. Have a set of sunglasses with you for when you come off the field. I've been hit, off the field, in the sunglasses from stray shots coming off airball bunkers. Saved my vision a couple times.
Torso: Ref shirt of your field's choice. If your field uses the "zebra" ref shirts/jerseys, that's perfect. If they have yellow/orange/bright shirts with their logo on them, great. If they don't specify, get yourself a bright t-shirt at Walmart for under $5. Get a couple of them. Maybe even spray-stencil "REF" in big letters front and back if you're even slightly artistically inclined.
In cooler weather, layers will be important, but be sure you can strip them off as you move. As a ref, you should be covering far more ground than the players.
Arms: Playing arm pads. You're going to get shot, maybe a lot. Might as well be padded for it. Padded gloves aren't a bad idea, either.
Legs and such: Playing pants and pads. Again, you're going to get shot, and you also need to be able to move quickly. Most playing pants are ventilated enough to give you decent airflow, which is nice. In cooler weather, you're likely to have enough room under there for jogging pants if needed.
Can't stress this enough, going back to the idea of "you're likely to get shot, probably a lot". WEAR A CUP. Not a soft cup, a real one. Yes, it sucks. Yes, it chafes. When that renter lying in wait/ambush decides to "test fire" into your gentleman's bits at a range of five feet, you'll be very happy you've protected your parts. "Gee, ref, sorry."
Feet: Waterproof. Ankle support. Comfortable. I've taken to wearing Gore-Tex combat boots. You may well wind up not needing that much support, but when you do, you'll tell the other refs how glad you are to have dry feet at the end of the day.
Addendum: Bring a change of clothes to casual wear for after gear's turned in. T-shirt, hat, jeans/shorts, socks, underwear, and casual shoes. It sucks having to drive home in stinky ref wear.