I'm not an expert, but you should definitely study law, criminology, some psychology, forensics, and some chemistry.
If you want to be a detective/officer, law, criminology, and psychology might be the best choice.
If you're into scientific analytical jobs, forensics and chemistry is probably the best.
Some more useful topics include physical science engineering, language proficiency, finance/accounting, and computer science.
And overall, you should probably have a healthy physical :P