Watch the video and read the text for information on how we start notes on the clarinet.
There are multiple schools of thought on how we should start notes on the clarinet. The most important part is that you make contact with the reed with your tongue at the beginning of every note.
One approach is the old saying, "top of the tip of the tongue at the top of the tip of the reed."
Another approach is to imagine saying the word "dee" or "knee" when tonguing. Add the mouthpiece and make contact with the reed while thinking about one of those words.
A simpler approach is to simply touch the reed with your tongue using as little effort or motion as possible. Work for a percussive and clear beginning to the note. Let the sound be your guide. Try different places of contact with your tongue and with the reed.
You may hear some teachers mention anchor tonguing. Anchor tonguing is starting and stopping the vibration of the reed with your tongue while maintaining a steady air stream throughout. This is demonstrated in the video above. I do not encourage anchor tonguing, but I want to make sure you know what it is.
The two most common mistakes students make with tonguing is that they either tongue too hard, or they don't tongue at all.