Once you've gotten your bike, you'll need some gear to protect you from the elements and keep you safe on the trail.
For clothing, padded shorts will keep you more comfortable on long rides. Add a waterproof jacket in case the weather changes and a reflective jersey if you're going to be riding at night. Knee and elbow pads will help shield you against scrapes and falls, and gloves will keep your hands warm and protect against blisters when you grip the handlebars.
Choose a helmet that's been certified by a safety organization such as the Consumer Product Safety Commission. The helmet should fit snugly, and it shouldn't move when you tilt your head. Extra vents will increase air circulation to your head, keeping you cooler.
If you have clipless pedals, you'll need shoes that fit into your pedals' cleats. They should be comfortable, durable and have a hard sole to help you pedal more efficiently. Add thick cycling socks to prevent blisters.
A good pair of sunglasses will protect your eyes from the sun's ultraviolet rays, and keep out dirt, wind and bugs. Choose shatterproof lenses with high UV protection.
Water is a must-have so you don't get dehydrated. Make sure you take a drink every 20 minutes or so, even if you aren't thirsty. If you don't want to continually pick up a bottle while you ride, get a backpack-mounted system. Also, bring along a snack, such as an energy bar or piece of fruit, to keep you going.