An ADN and BSN have the same nursing responsibilities as far as patient care, but with a BSN, there are more job opportunities in supervisory positions. I am in an ADN program now, but plan to get my BSN after I get some work experience.
Different hospitals, and sometimes states, may have programs for tuition reimbursement, or to pay your tuition if you sign a contract to work for a specified time period. For example, in NC, the state has a program that helps with school costs if you agree to work full time within the state for a specified time, but it is based on financial need. These programs vary, so you might be able to find out by calling the hospitals themselves, or the financial aid office of a local college.
See your guidance counselor in HS to help with what classes you should take, but from my experience, you will need algebra, biology, anatomy, physiology, chemistry, and any type of human relations electives that might be offered.
As far as taking college classes in HS, I guess it depends on what you think you can handle. If you are able to take them, then it probably couldn't hurt!
Hope I've given you the info you're looking for, maybe someone with more experience can explain in more detail. It's been a while since I was in HS! Good luck!