1. First of all, the fact that your heart pounds to the point that you can feel it, you must acknowledge that there might just be a little tiny bit of anxiousness / nervousness taking place. This is the first and most important step of all. Just acknowledge it silently in your mind.
2. Next, try to find out why you're anxious when being called in class. Are you afraid of being wrong? Being laughed at? Being the centre of attention? Being scrutinized on your answer? Or your appearance, clothing? Or maybe it's just simply a matter of not getting used to?
3. Tell yourself that it's normal, other people would probably feel nervous too when they are called upon. If your concern is the answer/response, as long as you're prepared, assure yourself you will do fine. If it's because of other above-mentioned reasons, just be objective and tell yourself what matters is just “your answer”. Just do or say what you have to. All other things shouldn't matter to you. In any case, you can't possibly control what other people think of you anyway.
The most important thing to remember is: if you give a wrong answer, the answer is wrong, not you. It says nothing about you except that you have the guts to answer a teacher's question with something besides "I dunno."
Teachers prefer that you raise your hand to be called on, even if they don't require it. Then they will be likelier to be nice to you, since you were nice to them.
4. Continually enter environments and situations where people DO CARE what you think: by giving small talk on a subject in front of people you are close to.
5. Ask question at the beginning or start of a class. After a few questions the inhibitions will die out. whenever I meet someone more intelligent, wiser or interesting I ask a lot of questions. As a result, I learn so many new things.
6. If your younger sister had the same problem what would you suggest her? Follow the same advice.
7. Pay attention to your physiology. When you notice feeling nervous, pIease check how you are holding your body. Usually, you would be slouching, and your breathing will be quite shallow. Hence, straightening up your posture and breathing will deeply help you feel more confident.
Note: There is actually scientific research that says that the way you hold your body gives you more confidence.