A few things to check (assuming you have never flown before):
- Check that the rudder and elevator move the correct direction. Standing behind the model when you move the stick right the rudder should move to the right and vice-versa. When you pull the stick back toward your body that is up elevator, the elevator should go up, and vice-versa.
- Check that the model balances as per manual.
- Check the prop is on the correct way round (writing on blades usually faces front).
- Make sure all flying and control surfaces are straight and twist free.
The most common problem is pilot error and/or a bad hand launch. If you can find a nice smooth surface try a roll off ground take off. Always take off directly into any breeze (don't fly if it's windy). Allow the plane to gradually build up speed and focus on keeping it tracking straight with the rudder. It will lift off itself once it's up to speed. Use very gentle control inputs.
If you have to do a hand launch then it's best to get someone you trust to launch for you while you concentrate on flying the plane. Launch should be firm but most important it must be straight and level, directed toward the horizon, not upward
, it's more of a firm 'push' than a throw. Most people who have never launched a model plane before try to throw it like they would a ball, steeply upward and with a wrist action that makes the model roll over. Allow the plane to build up speed while flying level then very gently climb away, do not haul back on the elevator or the plane will loose airspeed, stall and crash.
Best advice I can give you is to find an experienced flyer in your area to help you out with the first few flights, there is no substitute.