I drive a stick shift car and an automatic scooter, how hard will it be for me to learn to ride a motorcycle?

If you are proficient at both, you shouldn't have much of a problem. The main difference between a motorcycle clutch and a car clutch is that motorcycle clutches are bathed in oil, and are very difficult to burn out. You don't have to worry about "riding" the clutch like you would in a car. The principle is the same, Much of slow speed motorcycle operation happens with the clutch partially engaged- this is the friction zone - and that is perfectly OK. You will have better control over the clutch with your fingers than you do in the car using your foot.