In order to be a nurse, you can start from wherever you are, educationally and in terms of your life experience. Some people find that it is helpful to their nursing school applications to do some volunteer work in a hospital or to take some science or math prerequisites, but you might be able to get into nursing school (depending on your background) right now.
Generally speaking, there are several ways to become a nurse:
1. You could go to a two-year (community college) program, get your Associate's Degree in Nursing (ADN), and then be licensed as an R.N. Eventually you will probably have to do some kind of "degree completion" program so that you can advance in your career and/ or your nursing education (like to get a Master's degree, to become a nurse practitioner, or to become a nurse practitioner).
2. You could go to a four-year (university or state college) program, get your Bachelor of Science in Nursing BSN) degree, and then be licensed as an R.N.
3. You could complete a hospital diploma program and then be licensed as an R.N.
There are some other alternative methods, like going through an "Accelerated BSN" program (more on that here)-- that usually requires that you have a Bachelor of Science degree in another science-related field.
Once you complete some form of nursing school and become licensed as an RN, you will get your first job, where you will "specialize" and receive on the job training in fields like cardiology, pediatrics, and oncology.