Use a properly fitting kayak for both comfort and paddling efficiency. According to REI, it is important that the seat is comfortable, the foot pegs can be adjusted to your body size and the cockpit area allows enough room for you to get in and out easily. You will also need a paddle and access to a spare paddle as well (either stowed in your boat or another member's boat). You'll also need a spray skirt in any whitewater situation; it covers the area below your waist and wraps around the cockpit of the boat to keep water out of the boat.
A personal floatation device (PFD) is required by law for any kayaker in the water. PFDs keep your head afloat if you fall out of the kayak, but the PFD must be properly fitted for your body size. You'll also need a helmet to protect your head from rocks when you get turned upside down in the kayak. Also, every member of your party should have a rescue rope bag, which you can use to help kayakers in trouble or tow other kayaks.
Dress for the temperature of the water rather than the temperature of the air since it is guaranteed that you will get wet. Like most other sports, it is a good idea to dress in layers. The standard dress for kayaking is an inner wicking layer followed by an outer layer such as a dry top, wetsuit or paddle jacket to protect from the wind and water. Also, some kayakers like to wear gloves to protect their hands from the coldness of the water as well as prevent blisters from gripping the paddles.
Don't forget to pack sunscreen, lip balm with SPF, sunglasses and energy food. REI also recommends that one person in your party carry first-aid supplies, a backup water treatment system, signaling devices such as a whistle or mirror, a headlamp, and a good multi-tool. Store these personal items in a waterproof dry bag stowed and tied to the inside of the boat.