You might want to ask your guidance counselor if there is anything available. I know in NY State, there are various internship opportunities at the local city council member's office or a state senator's region. I wouldn't recommend sending a letter randomly to a company because they usually turn a blind-eye to a high school student. However, I DEFINITELY suggest that you find someone who has connections to one of the fields you want to intern at. It makes the process a lot easier because you'll have a direct source to him/her.