what causes my civic to shake, like really shake bad when i am on the highway?

If it is front tire related you would feel it in the steering wheel, if it is in the rear wheels the whole car would shake at 45 to 60 mph. Look at the condition of the tires see if they are worn uneven or have damage/sidewall damage. It's most likely tire related but could be worn suspension components. An engine misfire would cause rough running chugging at higher RPMs.

More than likely the tires are badly out of balance. There could be loose parts on the front end. The best way to find out is to have it inspected properly.