Everytime i start my cardio workout, i get lightheadedness and mild headache and sometimes moderate pain in my left shoulder. what could be the cause?

Which problem?: Are you asking about exercise intolerance or pain in the shoulder? Unless you have baseline medical problems such as diabetes, congestive heart failure, muscular dystrophy, juvenile arthritis or some other unusual malady for a 17 year old then, I'm sure answers to your questions are very simple. You will just need a little guidance and direction as to what and how you're doing things.