Check your manual. For most trainers today, a good rule of thumb is to set the center of gravity so the plane sits level or very slightly nose down with the plane balanced on the wing spar. Slightly nose heavy will make the plane more stable. As you advance, you'll find that the nose heavy condition may require more up elevator as you slow down, which puts you closer to the stall condition, but the trainer airfoils tend to be pretty forgiving.

NEVER fly tail-heavy. (I know, some 3D aerobats like to be tail heavy to improve the hovering, but this is the beginners forum). A tail heavy plane is inherently unstable. The plane will want to correct itself to the point where the center of gravity is in front of the center of lift. That means the plane wants to flip backwards. Every elevator input will be magnified, and the plane will abruptly tip up or down with no input.

If the fuel tank is in front of the desired balance point, as it is with nearly all planes these days, balance it with the tank EMPTY. That way as the tank drains, the plane becomes less nose heavy, but will not become tail heavy.

Good luck, and have your instructor check out the plane before you fly (HINT HINT HINT).

Brad