If you are a beginning clarinet player who is just learning how to play notes and read music, then you must also learn about the register key. Found above the hole where you place your left thumb, it makes the clarinet jump to a different set of notes in a higher octave.
Learn to read music. The clarinet plays notes in the "G" clef. Take the time to learn the notes as they appear on the music ledger. There are a couple of sayings, which will help you remember the names of the notes. For the "lined" notes, people often say "Every Good Boy Deserves Fudge." The capital letter of each word refers to the lines on the staff from the bottom up. E, G, B, D, F.
Also a word to remember the spaces on the staff is FACE. The corresponding notes are F, A, C, E. Memorize these in conjunction with the staff as well as learn the notes (they simply repeat in order) above and below the staff.
Educate yourself on the fingerings for the lower register of the clarinet. Forget about the register key for the moment. Take the time to learn the basic fingerings. "F" is simply covering the thumb hole. "E" is the thumb and first finger. "D" is the thumb, first and second fingers. "C" is the thumb, first, second and third fingers. "G" is the open position where every hole is uncovered. Continue to learn the rest of the notes in the lower register by studying a Bb clarinet note guide.
Press the "register key" found just above the thumb hole on the back of the clarinet. This is all you need to do to enact the upper register. Note that, unlike other instruments where the register key makes the notes simply an octave higher (and therefore the same fingerings) the clarinet is set an octave plus a fifth higher. Therefore, you will have to learn entirely new fingerings for the upper set of notes. Consult your Bb clarinet note guide book.
Release the register key to go back to playing notes in the lower register.
Practice playing notes in the lower register and then continuing up onto the higher register without stopping. The most skilled clarinetists can do this with ease. Make sure your reed has no chips or cracks when you play. If so, you might end up making some high register sounds you don't intend to, like bad squeaks.