The basic principle to know is that the center of gravity must be forward of the center of pressure for the rocket to be stable. CG is easy to find. It’s where you rocket balances if you hold it on the side of your finger. CP is a little more difficult to find without doing some math or having a simulation software. The old school, paper cut out CP method will get pretty close for simple designs.
Center of gravity is the point on the rocket where mass of the rocket is equal on both ends. Center of pressure is the point on the rocket where aerodynamic forces are equal on both ends.
Hopefully That makes sense, Read the Apogee article and ask questions. There is a lot of knowledge on this board.
Because no one ever accused me of restraint when it come to the beating of horse corpses, there's also a lot of basic information at NASA Glenn's web site. It isn't loading for me at the moment, but here's a URL for one oftne pages, and it should have links for the rest.
Is it more advanced information you're after? Without going into it, I'll just there is a lot more you can learn, which is usually of little importance in the hobby. To squeeze every bit of performance out a rocket, especially if you're getting into high power, you may want to get into it.
Most model rockets don't "fly". They don't even "glide". They fall from the moment the motor burns out in a near-parabolic curve. If they are lucky, deployment events interrupt the fall with other falls that are on a very different curve.
Those other falls have similarity to a "glide" in that the velocity can be static. If the booster separates from the top of the rocket via shock cord failure, and has a fair amount of fin in the right areas relative to the CG of this lower part of the rocket, the booster can glide on the way down. Models of various missiles have been witnessed gliding backwards after breaking the cord, but this is still variety of failure.