I'm applying for university and I wonder if my cancer would ruin my chances of getting in? Another thing is it consider a disability? How should I go about mention it?
It will not affect your chances of getting in.
Every cohort of freshmen will, as any other group of humans, include students suffering from chronic disease. Some disclose their medical conditions as a way to show the admissions committee how they overcame adversity. Some don't disclose it. For those that disclose it, I've never heard of any negative comment against the prospective student suffering from a chronic disease, including cancer.
Students suffering from cancer also have, as you'd expect, a diversity of diagnoses and severity of cases. But I've seen the most severe of cases, e.g. stage 4 cancer, being admitted to highly selective universities, even though the student was surely not going to finish his degree, which indeed is what happened. Most cases of cancer patients I've seen admitted to the university are from students whose cancer is in remission. Again, this is to show the range of cases admitted I've seen, although I don't have any data to support this claim, and I don't know of any reliable data publicly available.
It is also common for students suffering from chronic health issues to receive accommodations. For example, students taking chemotherapy need to take a few days off each month, and the university produces a letter to professors requiring that accommodations be given (e.g. deadline extensions.) Professors are not told the nature of the condition nor are they allowed to ask. This is good, as you don't have to explain yourself to each professor every semester. If this is called a "disability", it will depend on the jurisdiction, but at least as it pertains to the professor/student relationship, it does not matter, as accommodations do not require a disability.
I wish you good health.