You will fall off your mountain bike at some stage. But do let this put you off. You will not be speeding down the trails on your first couple of rides so your falls will tend to be of the comic variety and will be far less painful than you expect.
If you do take a fall try not to get nervous when you get back on the bike. The more relaxed you are, the less likely you are to take a spill. A rigid body will result in jerky movements on the bike which never end well. So think happy thoughts and off you go!