Before you cancel your car insurance policy, you should first buy a new policy, assuming you still need to insure your car and you are switching car insurance companies. Then, immediately contact your current insurance company to cancel your old policy.
Often times, companies require you contact them in writing, but it's worth the time to call first and confirm your company's process. The company may be required to report the cancellation to the state department of motor vehicles.
To get a refund on the old car insurance policy, make sure you communicate your end date to the car insurance company. You may be charged an early cancellation fee, but other than that, you should receive all the money you are owed.
You absolutely should not wait to let your current insurer know that you are canceling your policy. If not, they will likely start a new policy just as your current policy expires.