Your cycling kit
Ideally, this would be our fancy RCCC Kit, but it is not required. It is highly recommended that you bring a pair of padded shorts and a cycling jersey. These will keep you comfortable (top and bottom) over the duration of the ride. For colder rides, you may wish to bring layers and/or arm/leg/knee warmers. Keep in mind, as you ride you may warm up and these will need to be packed away.
Spare Tube / Puncture Kit
It is inevitable that you will get a puncture while on the road. It happens to everyone at some point, sometimes multiple times per ride. Make sure you have a spare inner tube that fits your tire, tire levers to get your tires off and some way to re-inflate your tire such as a hand pump or a CO2 cartridge.
Emergency Contact Info
You should be able to reach out to the ride leader or another contact in the event of an emergency. A cell phone may not always stay charged, so it is good to carry this information in hard copy somewhere on your person or bike.
On most rides, you will need water and food. Nothing is worse than being an hour away from the nearest town and running out of energy. Plan on one bottle of water for every 45 minutes of activity and torching a few hundred calories per hour at a minimum. It is highly recommended to bring an energy bar (~250 cals) and a few gels (100 cals each) to any ride over two hours. Bananas are great portable energy too. Eat them before you get hangry (a combination of hungry and angry, which happens when you wait too long to eat).
For early morning rides, rides in the evenings as daylight fades and quite frankly to continue to be visible on the road, you should have lights for your bike. In most circumstances, a white front light and a red rear light will suffice. However, if you are riding pre-dawn, or in dusk, you should have a front light with enough power to allow you to see road hazards (500-800 lumens).