Bad tires, tires unbalanced or not at the right air pressure , or you need a four wheel alignment! Could be some suspension or drive axle problems too. Get an alignment and they will know what it is. Mismatched tires?
The problem could be a bent tie rod or worn out CV joint or even low air in one of the tires. Start by checking the air in the tires and ensure the PSI is correct and test the car. The engine mounts can also be worn and the shake is the due to the usual engine running but poor constancy.