If it only happens when you are braking I would suspect warped brake rotors. Other reasons the front end could shake would include unbalanced tires or the vehicle needing realignment. Whatever the case may be; driving a car in this condition is unsafe and you should repair it or have it repaired before you drive it again.
Wheels are out of balance
Posibilities are During braking Warped brake discs (rotors) Worn suspension components Excessive play in a front wheel bearing During normal driving Wheels out of balance Tyre out of shape damaged road wheel.
If it varies with driving speed, look for an out-of-round tire. If it vibrates only when braking, you probably have a warped brake rotor.
might have misaligned or loose wheels
Wheels require balancing.
You may need a wheel alignment, or you may need bushings.
Brake rotor/rotors are out of round / warped. Check rotors and REPLACE as needed.
Warped rotor and worn out brake pads.
Wheel balance may be off...
It needs an front end alinement or your tires & wheels need rotated
This would normally be down to either: Warped Brake Discs (in some counties these are called Rotors) where there is a small ripple in the surface of the disc. This causes the brake pads (which are flat) to make intermittant contact with the surface which causes a vibration. This "warping" is not always visible to the naked eye. Worn or Perrished suspension compnents. This can lead to the wheels moving around in an unctrolled manner during braking which can also cause vibration.
If it happens when you are driving fast (60-70 MPH) it could be that your wheels are out of alignment.