What are some ways to reduce nervousness about seeing cardiologist for first time for a teenager?

Straight talk. Very often, anxiety (about anything) is made worse by reluctance to voice one's fears openly. It's a kind of holdover from childhood thinking when we imagine that by not seeing something it actually disappears. Most people find that when they get over this "fear of fear", and express their worries, the anxiety diminishes. If this is so for your teen, make yourself available--tho w/o pushing.
Stay active. Exercise is a great way to relieve stress. Your body releases endorphins during exercise, which can help you feel calm. If you exercise 30 - 60 minutes a day, your stress levels can improve. When stressed, take 5 slow, deep breaths with your eyes closed, then roll your shoulders forward 5 times, then back 5 times. This will slow your heart rate and release tension in your neck and shoulders.
Reassurance. Is the best way. You are going to meet a highly trained expert who will partner with you to determine if there is any concerning issue & if present treat it....You will be going from a state of unknown to known.