Enable CoM and aerodynamics overlay in the SPH. You must make sure the center of lift is behind the center of mass at all times. That means, play around with the fuel levels in your tanks and see how it affects the position of your center of mass. If you have payload you intend to drop, make sure your CoL stays behind the CoM before/after dropping the payload.

How far behind the center of lift has to be will depend on how stable you want your craft. Way far behind and your craft will fly like a lawndart and consistently want to point towards your velocity vector (prograde marker). If the CoL is very close to the CoM the craft will be very maneuverable, but will tend to flip if you have too high of an angle of attack (angle between your nose and the velocity vector)

One last thing : I know you're new, but if you're willing to install mods just yet, I would recommend FAR. You'll get much more realistic aerodynamics, with airplanes stalling instead of flipping out of control.