This achievement is fairly simple to accomplish IF you can find the correct vehicle with which to do it.
When doing a wheelie with most motorbikes in GTAIV, you need to pay attention to the balance of the bike. Too much speed will cause you to flip over backwards, while too little will cause you to lose the wheelie. There is a bike in the game called the Faggio, which is basically a Scooter. This scooter cannot flip backwards, which means all you need to do is get the initial wheelie and not crash.
The Scooter is reportedly located around Middle Park, however it can also be obtained as part of a side mission for Stevie. Once you have the bike, the wheelie can be completed fairly easily on most bridges, or at the airport, or even on Alonquin Island if the traffic is light.