Can I become a criminologist with a criminal justice major? what if my school doesnt have a criminology major?

That's a start. To be called a criminologist, you're really looking at master's or PhD level research.

Bachelor's degree does not qualify you as a "criminologist" to begin with. And "criminal justice" is a fad within sociology, social sciences. Not worth much as a major. Not worth much on the job market. And definitely does not make you a "criminologist."

Are you talking about the broad definition of criminology or the focused professional definition?

If you are talking about the broad definition, get yourself a job as a crime lab technician after you graduate.
Some criminologists have a very narrow focus, such as accountants who focus on business and tax fraud.

If you are talking about the narrow definition, don't worry because you will need more schooling after you get your degree.