The most common mistake a beginner has is approaching the flute with a rounded embouchure; where you form your lips as if you just bit into something sour - your lips contract together from all sides as if you are whistling or blowing over a pop bottle. This formation leaves too large of an opening to control the sound.
Here is an exercise I teach that you can do without the flute. Put your lips together, then as if you are grinning, pull your lips back. Now, different than grinning, don't turn the corners of your lips up, but pull them straight back. Now, with a controlled breath, push a stream of air out your lips. Your lips will part under the pressure in a certain place. Take your finger and locate where the air flow is coming out. That spot is where you will have the most air control. That is where you will want to put the opening on the flute., The beauty of this technique is that you can practice this anytime anywhere to improve your sound.
Hey, and don't even get me started about whistle tones. - Ralph