First, make sure the CG is properly placed, shift equipment or add weights until that is done. Make sure that the wing is properly seated and fastened down. Was the motor already mounted when you got it. Check it for alignment. You should have about 3 to 5 degrees downthrust (using the elevator as reference) built in and about 3 degrees right thrust. If not, adjust by adding washers under the appropriate engine mounting bolts. Now, get it up to a safe altitude, set the throttle at about mid range and trim it out for level flight. Now add full power. If it immediately zooms up, you need to increase the down thrust, if it tends to dive as you increase power, reduce the down thrust. Now, it will probably climb as you add power but the transition to climbing from level flight should be gradual as the speed builds up. If you get the thrust line exactly right, it will continue to fly level as the increased lift due to increased speed is exactly countered by the increase in the downward component of the thrust line. I can usually get all my planes trimmed out so that there is little or no change in altitude with engine thrust as power changes from high idle to full bore.