The absolute minimum that you need to know if you want to fly safely is the following:

1. Weight of your rocket
2. Thrust of your selected motor
3. Center of Gravity of your rocket [CG]
4. Center of Pressure of your rocket [CP]

At organized launches be prepared for the Range Safety Officer [RSO] to ask you about these items.

WHY?

In order for a rocket to be stable in flight the CG must be forward of the CP.  The CG is the actual pivot point around which the rocket will turn.  The CP is the point at which all aerodynamic forces act.  If the CP is behind the CG, any turn the rocket makes will cause the airflow to return the rocket to its previous heading.  It's easy to visualize, just think of an arrow.  The feathers in the back keep it stable.  If you were to move the feathers to the front, the arrow would no longer fly straight.

Knowing the weight of the rocket and the thrust of the motor is also important.  Since model rockets do not have active guidance they must be moving fast enough when they leave the launch rod (or rail) in order for sufficient air to flow over the fins for stability.  As we will see later there is a correlation between the thrust and how fast the rocket accelerates.  The standard rule of thumb for a safe flight is that the motor should provide a minimum of a 5:1 thrust to weight ratio.  So a 1 pound rocket should have a motor that provides 5 pounds of thrust.

So that wasn't so bad, now was it?  Before we go off and delve into the advanced stuff lets see how you get this info. 
--Weight of rocket, we use a real complicated method here, you put it on a scale.  Make certain it is fully loaded for flight, we want the weight with motor, parachute(s), any electronics etc.  This is also called its all up weight.
--Thrust of the motor, this information is available from the motor manufacturer, usually in the form of a thrust curve.  You may need to convert the units so that your rocket weight and the motor thrust are in the same units. 
--CG is simply measured by balancing the rocket, the point where it balances and remains horizontal is its CG. 
--The CP needs to be calculated, however most kit manufacturers provide this information with the kit.  It is generally measured from the tip of the nose back.  If your rocket is scratch built or a modified kit you will have to calculate the CP or use a program such as RockSim in order to find it.  A quick and dirty method is called the cardboard cutout method.  Simply trace your rocket's outline onto a piece of cardboard.  Cut it out and then balance the cutout.  The balance point is roughly where the CP of the actual rocket is.

Now the one remaining thing you'll more than likely need to know is what is a Newton?  Many rocket motors list their thrust in terms of Newtons.  For example if we look up the reported specs on an Aerotech J350 reload we are given a maximum thrust of 923N and an average thrust of 395N.  To the average American those numbers mean little, we are used to pounds.  Newtons X 0.2248 = Pounds
Maximum thrust of a J350 is 932N X 0.2248 = 210 pounds of thrust
Let's try that again
Average thrust of a J350 is 395N X 0.2248 = 89 pounds of thrust

If you're thinking ahead you should be asking why a difference between maximum thrust and average thrust?  That will be answered on another page yet to come.

Warning... The equation pages coming up will not be the easiest to read, HTML can do many marvelous things, but it is a rotten equation editor.