If you find that your CG is not in the correct spot, try moving the battery and/or the reciever around until it balances correctly. Hopeflly this is all you will have to do. If moving those doesn't do it, you may be forced to add a little weight. Hobby shops sell stick-on lead weights just like the ones used for tires. Don't rely on the weight's sticky adhesive though. I usually epoxy the weights inside the plane or secure them to the outside with screws. Most of your beginner type planes that come from reputable manufacturers do a pretty good job of explaining this stuff in their manuals.