When i drive at 100 kph my stering is shaking.why?

Hi James,the problem could be one or more of your wheels,,especially the front ones,could be unbalanced.If not,then one or more of the steering linkage members could be worn.To establish the latter,jack up one of the front wheels at a time and try rocking the wheel by holding it side ways looking for play.For the former,you'll just have to take the wheels to a shop that has a wheel balancing equipment.Good luck buddy.