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.
check yr stabilizer bush
Check pLugs and wires and the vibration my be from brake pads or warped rotors