You can really major in just about anything in undergrad, as you have to go on to law school to become an attorney. I always recommend two things to people who want to be attorneys.
1. Think about what you want to do if you end up not being a lawyer, and major in that. This is because there are a lot more people graduating law school than there are jobs for them. I don't want to discourage you, but you need to be aware of this. Law school is expensive, and you need to be able to get a job as a lawyer to pay back the loans!
2. Consider the sciences. There is lots more to being a prosecutor than simply understanding crime and criminals. For example, let's say you are prosecuting a DWI homicide case. You should understand the basics of lighting, weather, tire tread, road surface, reaction times, etc. There is a lot of science in many crimes. There is also a growing need for understanding technology.
Whatever you major in, you will have the opportunity for electives. Get involved in speech/debate, and take a logic course!
Best of luck!