You can not use your glasses prescription for contacts. Contact lens and glasses prescriptions are very different. A contact lens eye exam looks at the shape and size of your eye so it measures things like the base curve and diameter. In addition, since a contact lens sits directly on the eye, the prescription must account for that.
Glasses, on the other hand, sit just off your nose so the prescription is adjusted for the difference between your eyes and the frames themselves. If you were to use the same measurements, then your contact lens would be much too powerful than what you actually need. This could cause dizziness and headaches. If you have a prescription for glasses and would like to try contacts, visit your eye doctor for a new exam and contact lens fitting.