I'm going to tell you a story about one of my friends, named Jimmy. Jimmy didn't really do much in his spare time,and got really bored over the summer. So he started looking for new things to keep him occupied. After a long while of searching, Jimmy found out that a bunch of his friends were biking these really cool trails in really cool places. Jimmy liked this idea, so he packed up his bike and took it off to a nearby trail. He'd done everything he needed, or so he thought. He brought a helmet, and made sure his tires had enough air. Everything looked good, so Jimmy started riding. About half an hour into the ride, he felt his arms were at an uncomfortable position with the handlebars. Jimmy didn't have the proper tools to fix this so he just kept on going. About an hour later, Jimmy noticed that his front tire was becoming a little flat. (he must have run over some sharp glass on the trail from them darned partying youngsters.) Jimmy didn't have a spare tire or any of the tools needed to change a tire so he ignored it and kept on going. Another half hour later, the reality of everything happpening was just setting in. He had a flat tire, his arms were so sore, and he hadn't had anything to drink or eat for hours. Jimmy had already gone very far. He couldn't make it back on his own. He didn't know what to do so he just stopped, and waited, hoping somebody might come by and be able to take him back. But nobody came... Jimmy died out there.
The End
Now, obviously, this is just a story. I don't actually have a friend name Jimmy, and none of my friends died while bike riding,don't worry. But I'm sure that somewhere in the world, something similar to this has happened; someone died or got awfully close to it because they weren't prepared while bike riding. So to prevent this from happening, I'm here, making this list of some of the most important things to bring on a bike ride. 
(These are in no particular order.)
Number 5: Water. Water is important in any circumstance. Not having water with you could be very dangerous. You've got to stay hydrated, especially if you're biking hard and it's hot outside. That one's pretty self explanatory.
Number 4: Snacks. This one kind of goes hand in hand with water. You need to bring something to eat when you go biking for energy. It's good to eat before, during, and after a ride, I'm not saying you have to bring a full three course meal with a huge turkey and mashed potatoes. Just bring something small and nutritious, like some fruit or a granola bar.
Number 3: A spare tube/tire. Nothing sucks more than having a flat tire or busted tube. Always make sure you bring an extra, just in case.
Number 2: A friend. Riding on your own is alright in some occasions, but it's usually best to have atleast one other person with you. Not only does a friend make bike riding a little more fun, but a friend also makes bike riding a little more safe. You don't want to be left out there all alone if you fall off your bike and get injured. If you bring someone along, then you always have someone who is able to help you.
Number 1: A good bike. Before you go on a ride you should always make sure that every component on your bike is in good shape. You don't want your bike breaking on you every time you go out for a ride. You also need to make sure you have the right bike for the type of ride you're going on. You don't want to bring a road bike with skinny little tires on amountain bike trail, and you don't want to bring a BMX bike to a road race. 
That sums up my list of things that you should bring on a bike ride. Be safe out there!