To become an Oncologist, or cancer specialist, a medical degree is the first requirement and usually takes five (5) years to earn. After that, a one (1) year internship and several years (generally two (2)) of residendcy in oncology will suffice. Of course, rapidly developing strategies, medicines and treatment options require continual on-going education to properly practice in the field of Oncology.
Good luck. Who knows, YOU might be the person who finally eradicates, or at least finds a viable "immunization" to prevent, what is, in all probability, mankind's most dreaded disease.
An oncologist is a medical doctor who specialises in treating people with cancer.
YOu would have to do all the usual med school stuff (6 years) then intern, then residency befire starting to specialise.
It should also be noted that there are now several sub-specialties of oncology which require their own post-residency training.