Normally it is a tire out of balance. Can also be a bent wheel.
A car's steering wheel will vibrate when driving if the car's alignment needs to be adjusted.
the motor mount is bad
Normally this is caused by a tire out of balance or a bent wheel.
I have the same problem with my '91 Dodge Caravan -- it is probably the transmission.
Your car needs an alignment. Typically around $100 or less.
Maybe there is something wrong with your car, you should get it checked out.
If other things vibrate as well as the steering wheel you might have a damaged motor/transmission mount.
Check your power steering fliud, if it is low this could cause the vibration you are feeling.
You either have no/low power steering fluid in your car, or there is some other problem with the power steering, such as a leak in the tube.
Vibration of the tire is caused by a uniformity parameter called "variation of radial force". Usually a vehicle's steering wheel will vibrate (called torsional nimble) at a certain speed. This vibration is caused by the harmonics of this radial force. Picture the side view of a tire with different size springs within it. The force of these springs cause a distortion of this force and you feel this distortion in the steering wheel.
As you drive the steering tightens. You may need to check your power-steering fluid or put air in your tires.
check yr stabilizer bush