In general, I'd say it's better to avoid murder or kidnapping for the first case. With a teenage protagonist, I think it's best to gradually escalate the level of threat she deals with. I know it takes a lot of suspension of disbelief for me when dealing with teen detectives, and I would like to believe that a heroine is smart enough and cautious enough to appeal to a more professional authority, in a case as severe as murder. Theft is good, especially if it's of something important, sentimental or magical. Blackmail is also good, especially over magical secrets. There is also the Lois Lane style mystery of trying to uncover someone's true identity.