No, you won't flip forward, although it can be uncomfortable. Most times an engine stall at speed will feel like you accidentally hit the rear brake, and worst case--like a chain coming off and jamming the wheel and/or engine--you can skid. It's scary, but controllable if you don't panic.
In most situations, simply pull in the clutch, and if the engine doesn't start right away, roll to the side of the road and determine the problem. Usually it can be a little water in the gas, or lack of gas, a clogged jet in a carb, or something like that. If it continues and you have a bike with points, they are a place to check, since they do need periodic adjustment.
Mainly, don't let it worry you. Things happen, and when they do, we have to deal with them. And good luck riding; we all need that! 8^)