When I was in ninth grade, I started reading the Sherlock Holmes stories by Sir Arthur Conan Doyle.

You should be old enough to enjoy them and there isn’t anything that would be inappropriate for someone that age.

Most of the other mysteries that I know of would be beneath your age level (like Encyclopedia Brown stories) , or contain “adult” content that your parents might object to your reading.

The Cat Who… stories by Lillian Jackson Braun might also work. They are obviously written more for grown-ups, but there is nothing in them that I think would be overly “adult”. I would ask your parents first to be sure, though.

A good mystery story should have a good hook, of course, some situation that intrigues you from the start. It should have clues along the way that are subtle but not so subtle that the reader can’t find them and enjoy the fun of trying to solve the puzzle. There should be “red herrings” — suspects who turn out not to be the guilty party, but have motives for committing the crime. And there should be a surprise ending, one the reader doesn’t expect but finds satisyfing and logical. Needless to say, the reader shouldn’t peek at the last page, even though he or she may be sorely tempted to do so!