Why does my wheel on my 2005 2 dr Chevy Cobalt shake??

Ok.. Here it is.. I bought a 2005 Chevy Cobalt 5 months ago and Im having problems when i drive up to 70 mph and then slow down and brake, my wheel shakes.. Everyone has been saying that I need to balance my tires and need to get a tire rotation and that was the reason why the wheel is shaking...now I got it done today.. I ended up needing 2 new tires and I got my tires all balanced and rotated...when that was done....I drove my car and now my car wheel STILL shakes...does anyone know why??? another synopsis is that someone mentioned that I need a front end alignment...on a 2 year car..can that be??? please someone help and give me some answers!! thanks!!!