Greetings,
The most common reason for a car to shake when slowing down is a warped front rotor. The best thing you can do to resolve this issue is to take the front rotors to a brake or tire shop or even some auto part stores and ask them to turn the rotors. They will use a special brake lathe to flatten the rotors back into specifications and the vibration should go away. Without further description this is the best guess i can give you. Alternate items that could also cause undo vibration when braking would be loose inner or outer tie rod or issues with bearings.
A failing tie rod end can be dangerous and continuous driving of the vehicle without repair can start to effect other aspects of the front end.
Cheers!!
Moe