Just as a rough guess it sounds like you're giving too much up elevator, the plane climbs briefly, then stalls and drops to the ground. As Acarter indicated. Try using a very lite touch on the controls. Let the plane build up plenty of speed when you launch/take off. Then move the controls in small amounts and only for a moment of time. I tell new students to move the stick in small pulses or 'bumps', letting the stick return to center after each bump. This can help prevent over-controlling.

Of course, to fly well it has to be balanced at the correct CG (Center of Gravity) as Jetmech said.