Attach the clarinet reed to the mouthpiece first. The reed is a thin wood piece that you must purchase separately from the instrument. Wet the reed thoroughly with your tongue first, then slide it inside of the metal ring that's positioned around the mouthpiece. Make sure that the top of the reed is flush with the top of the mouthpiece---the position of the reed is key to playing notes on the clarinet properly. Tighten the metal ring when done.
Place your right thumb under the handle on the back of the clarinet and rest your fingers around the metal keys in front. Place the fingers of your left hand over the top row of metal keys and then allow your thumb to rest on the key located in the back of the top row of keys. Get comfortable with pressing down and releasing the various keys and covering the holes, one by one.
Place your tongue on the clarinet reed and make sure it's well-moistened. Hold your left thumb against the back key, then hold your left pointer finger over the first key on the front. Blow into the clarinet mouthpiece to play your very first note---make sure your cheeks are firm and lips pursed tightly around the mouthpiece.
Add your left-middle finger over the next key and blow again. Continue positioning a new finger over a new key (including the bottom row) to play different notes. As you progress in learning how to play the clarinet, you'll also learn how to incorporate the additional metal keys on the bottom and sides of the instrument to play specific notes.