Begin by creating a tight embouchure around the mouthpiece. The embouchure is the shape your mouth makes supporting the reed and directing the air. It takes considerable practice to build up the strength of a correct embouchure.
Blow through the reed to create sound from the instrument. Beginners may struggle even to make a sound for a while. Practice until you can make a sound consistently.
Place your tongue in the middle of the reed to stop the sound quickly. Find the "sweet spot" in the center of the reed, both vertically and horizontally to mute the sound most effectively. Placing the tongue too high on the reed can degrade the reed and put chips in it. Putting the tongue too low on the reed won't effectively stop the sound.
Continue to practice and use tongue exercise to improve articulation.