Helmet - Get one that is safety certified by DOT or Snell and fits properly. Wear it on every ride.
Leather Gloves - Nice to be wearing when the weather is cold. Nicer to be wearing when you're sliding on asphalt.
Eyewear - A good pair of riding sunglasses or goggles will reduce glare during the day and protect your eyes from bugs and road debris. Carry a pair with clear lenses for when the sun goes down.
Boots - Get a pair of sturdy, over the ankle, leather riding boots. You can always bring along a lighter pair of shoes for walking around.
Jacket - Go with leather or a Gore-Tex type of material. Body armor inserts are a good idea.
Riding Pants - Get a pair that's reinforced in the knees and backside. Some do it with material like Kevlar, others offer armor inserts. At the very least, put on some long pants, shorts are a no, no.
Sunscreen - A day riding in the sun can scorch your skin. The higher the SPF, the better the protection.
Bottled Water - The sun and wind combine to dehydrate your body. Don't ride thirsty and don't make the mistake of trying to use alcohol to rehydrate.
Tool Kit - At the very least, pack a pair of pliers, screwdriver, adjustable wrench and some electrical tape. Even if you don't need them, one of your fellow riders might.
Cell Phone - Today's smartphones offer everything from maps to weather. You can even make a call with them.