It may vary by state but the fastest way is to just take the test. But i would not recommend doing that.
There are classes you can take just like for driving. You can do them through the dmv i believe. but id recommend finding somewhere with a little more expertise, where riding is the teachers passion. I know the harley dealership by me does the class.
After the class you get your permit. With that you cant carry passengers, go on interstate, ride at night and have to wear a helmet. You then take the road test to get your full license.