Clarinet mouthpiece. If you purchased or received a clarinet and play the mouthpiece that originally came in that case, you are in for a big surprise. The dirty little secret is that most stock mouthpieces that come with the clarinet are horrible, usually very free blowing for the very, very beginner clarinetist.
BUYING A GOOD QUALITY MOUTHPIECE IS THE SINGLE BEST IMPROVEMENT YOU CAN MAKE IN YOUR CLARINET SOUND QUALITY. So, if you are financially strapped and cannot afford a clarinet now, search for some good mouthpieces. Clarinet-Now.com is a good place to start.
Starting with a good mouthpiece is important. I usually have my students try the Vandoren M-13 Clarinet Mouthpiece, Vandoren 5RV-Lyre Mouthpiece, and the Vandoren M-30 Mouthpiece.
Understand there are many, many, many more types of mouthpieces. But, these are a good place to start.