Yep, I agree with John. Just had the same thing going on with my van, you can have the rotors turned also but it is usually just as cheap to buy new ones, my mechanic said it is best to replace the brake pads at the same time.
Probably your rotors like the others said, but I think inner tie rods can also cause this if they have gone bad.
Disc thickness variation. Read this post on what causes it and how to fix it.
http://ricksfreeautorepairadvice.com/bra...
Your front brake rotors are probably warped and need replacing. Take it to a brake shop.
Every time I brake my car shakes and is getting worse.
