If you are interested in working in the field of hematology, you are going to have to achieve the same level of education as any other doctor. You will need a four-year bachelors or undergrad degree, then complete medical school and a residency.
Your bachelors can be in anything you want, however you need to make sure to take all the appropriate prerequisite courses before applying to medical school. Medical school should last four years, and your residency should be up to four years.
Once you finish your undergraduate, medical school and residency, you can begin training in the field of hematology. The training requirements typically take about ten years before you are completely done and a practicing hematologist.