The Tongue is used as an "air stopper" between the airstream (from the player's mouth) and the mouthpiece, a clarinetist never stops the airflow with her diaphragm, the air is merely interrupted by the tongue in a way specific to the musical context. It can be thought of as stones skipping over water, your tongue literally bouncing over air.
First exercise when we start tonguing is to set up the reed and the mouthpiece, as you normally would, but take it off the clarinet (with the barrel too) and begin to play long sustained notes allowing each to be about 8-10 seconds long, now when you want to shortly interrupt the sound, while blowing (playing notes) use the tip of the tongue and make the consonant sound of the letter "t" (if you find that "t" doesn't float your boat, use "d"). This is only a tiny interruption in the sound and should be approx 1/64 of a beat (a minute amount). You have now learned the basic legato tonguing, this is used in situations for example when you have a bar of ta's (Crotchets- For all those non-French note value people) that are just a bar of ta's, they are separated by a light tongue in an even manner. It would be appropriate to practice the tonguing of ta's or 2 beat notes (Crotchets or minums) with your mouthpiece/barrel setup and then move on to Tee-Tees (quavers, grouped into 2's) with the sound "t t" and learn to control the speed and dynamic of your tonguing. Then move on to music, put your clarinet together and play a song all tongued and then play it all slurred, and then play it as written. You will get better and find out how your tongue is most comfortable with practice.