Center of Gravity
Getting the center of gravity right in an aircraft is critical for it to fly well.
A common adage in aero-modelling is that nose heavy planes fly badly, but tail heavy planes fly once.
The center of gravity is the balance point of your plane. You will need to move your battery or add and remove weight from your aircraft so that it balances at the right point.
Check the instructions
Most planes will come with instructions on the right CoG (center of gravity) of the aircraft. This is often specified as a distance back from the leading edge of the main wing.
unfortunately it is fairly common for instructions manuals for cheaper planes to have an incorrect CoG listed. Try to confirm with on-line forums or someone who has flown the same plane.
Very roughly, you expect the CoG to be about 1/3 of the way back from the front of the wing. If you have doubt about the correct CoG then you should err on the side of making the plane too nose heavy instead of too tail heavy. A nose heavy plane may have trouble taking off and climbing, but it will usually be stable. A tail heavy plane may be very unstable and crash shortly after takeoff.
Check Online Forums
You should also check online forums for other people who have flown the same plane. It is very common for the correct CoG to be discussed.
Correct After First Flight
One of the key things you should check after your first flight is whether your CoG was in fact correct. Check your flight log and look at the state of the pitch integrator. The pitch integrator is in the PIDP.I field in your dataflash log.
If your plane showed a value of PIDP.I consistently above zero then your plane was needing to put in up elevator to stay level. Your plane is probably a bit nose heavy, or your elevator trim is incorrect.
If your plane showed a value of PIDP.I consistently below zero then your plane was needing to put in down elevator to stay level. Your plane is probably a bit tail heavy, or your elevator trim is incorrect.