What you want to be is called a HOSPITAL CHAPLAIN. Go to website: http://dictionary.com/
and type in 'chaplain' for a description of that job. On that page, to your left, click on 'hospital chaplain.' It redirects you to 'reference.com' where you can find more information and shows some websites where you can find schools to obtain your degree or certification.
There is a professional organization called HCMA (Hospital Chaplains Ministry of America). Founded in 1939, the HCMA is a nonsectarian, non-profit faith mission organization that recruits, trains, certifies, places, and oversees Chaplains in healthcare facilities and hospice programs throughout the world. HCMA is a growing agency sought out by both independent facilities and healthcare corporations (including acute care and long-term care facilities, and hospice). HCMA Chaplains serve throughout the globe.
On their website, you can click on 'facilities served' and it lists all the hospitals across the country that they are currently involved with.
As far as working with ONCOLOGY and KIDS, I did a Google search by typing in 'hospitals that treat children with cancer' and a lot of websites are displayed. You can from there probably find a hospital in your area that treats pediatric cancer patients.
I don't know what religious denomination you are, but the most prominent hospital I know of that treats kids with cancer is St. Jude Children's Research Hospitals which, of course, are Catholic. I went on their website and clicked on the 'Volunteers' tab. They need what they refer to as Eucharistic Lay Ministers who work with their Department of Chaplain Services. To do that, they require specific credentials from the Catholic Church.
You must be at least 18 years old to do most volunteer work at St. Jude's, They have a slideshow on the Volunteer tab describing all the different volunteer jobs you can do, as well as an Application Form you can fill out and submit online.
But you're only 15...they do have a program called 'Volunteen' for 16- to 18-year-olds. It has 2 sessions per summer and they select only 30 participants, each of which must submit an essay to be considered.
Miranda, I admire you for having such a big heart and a big dream to do one of the most important things on earth -- to comfort and minister to innocent children that are going through something terrible when they should be outside playing in the parks without a care in the world.
I hope I have helped answer the questions you had concerning how to find and follow the path you have chosen.