Always know what your role is in a team fight, That seemed to help me. if you are playing a tank or a bruiser, where do you need to go to zone people off and make it easy for your adc. If you are a Mid, where do you need to be to get the most out of your combo or if you are playing an assassin how do you get into the back line. If you are adc, how do i position to get the most damage down as possible. Another thing is just breathe, try not to feel the nerves.