wheelie n. lifting the front wheel off the ground, usually with some combination of pulling on the handlebars, pedaling harder, and balance.
From the Dictionary of Mountain Bike Slang
A Wheelie is the name of a move on either a bike or a motorcycle. It involves increasing power to the back wheel and leaning back. The result of this turn of events is that your front wheels leaves the ground and you begin to ride on just one wheel.
There are two types of wheelies on a bicycle, sitting and standing. The easiest to learn is sitting down. To do a sitting wheelie, start at a walking pace using a fairly low gear. You want to be pedalling at about 60 RPM, in other words medium slow. Now, while pedaling, lean waay back and pedal really hard.
At this point one of two things is probably going to happen. Either you're going to go up a bit and then come down again, or you'll pedal too hard and if you can't get your feet down fast enough, fall on your back. The trick is to lean back a lot and just practice until you find your balance point.
If you find yourself completely unable to stay up, there are a couple of things that could be going wrong. The first is that your seat might not be up enough. On my trials bike I'm unable to do a wheelie sitting down because my seat is very low. This makes it impossible to shift your weight back enough, and very difficult to pedal hard enough. If you can't keep you left/right balance, then read on.
The main disadvantage to wheelying while sitting is that it's difficult to keep your balance while stuck in a seat. When you stand up this problem is eliminated and it's easier to keep you balance.
To do a wheelie while standing, start off at about a walking pace. Try to get your weight as far back as possible, and then pedal hard. Keeping your seat down will reduce the risk of a bad testicle smashing and make this easier. What will often happen is that you'll rise when you first start, but as soon as you reach the bottom of your pedal stroke you'll drop. What your problem is, is that you need to have a pedal more smoothly. Try to stop pushing down on you pedals when they get to the 6 O'clock position and concentrate on pushing backwards instead of downwards. And keep your weight back.
With these tips and a lot of practice, you should be wheelying and outcooling all your friends with ease. Just remember practice practice practice. It took me about 6 months get to where I am now (I can stay up for max. 14 pedal revolutions). So if you don't seem to be getting anywhere don't be discouraged.