Until recently, a BS in a natural science like Biology, with a good amount of chemistry, has been the usual requirement to work as a criminalist or in a forensic laboratory. Now, many agencies will accept a BS in Forensic Science.
I suggest you look at the Criminalist and other Laboratory job openings posted on the Crime Scene Investigator Network to see what different employers require. If you identify an agency you would like to work for someday, do whatever they require to apply. If they require a BS in Biology, then get a BS in Biology, etc.
Would it be better for me to take a course in forensic science or chemistry, biology or physics if I want to work as a criminalist or in a lab?