To control the beginning and end of a note, the technique of tonguing is used. To prepare for this technique whisper the sound "taa". The sound begins with your tongue sitting behind your top teeth, blocking the passage of air, and you make the "taa" sound by quickly withdrawing it, and letting a stream of air begin from your outgoing breath.
Next, place the flute in the playing position against your lip and let your tongue rest lightly against your top teeth. Quickly release your tongue (like the taa movement), and the note will start. To stop the note, simply put your tongue back against your top teeth. The note will stop immediately, and your tongue will be in position to start the next note.