i have a 2007 titan and when i brake the front end shake really bad what could cause this?

warped rotor I would say.

How are the break pads? Have you jacked the front end off the ground and turned the wheels to inspect the tires? Have you checked for free play to eliminate a wheel bearing? If you are not savy in these areas I would suggest taking it to your local mike the mech and let him check these things. Might be the best 65 bucks you've ever spent.