Congrats on your choice of career, and know that there are a LOT of possibilities out there.
YES, focus on your prereqs, especially A&P. DO WELL. Many programs out there have a point system that ranks you based on several areas, one of which may be your grades. And for the benefit of ALL persons you come in contact with, PLEASE do well in any writing courses you take! It makes me cringe every time I see Nurses Notes full of poor grammar and spelling. A part-time job as an Aide or even a hospital volunteer can expose you to some of the choices out there, and give you a taste of things to come.
Take care of yourself. The Nursing Program is taxing, and if you don't manage your mental health (don't forget to do things for YOU!) you could wind up in a fit of depression very easily. Learn to take constructive criticism, to be assertive but not aggressive, and have confidence in yourself and your abilities.
Above all, BE OPEN -- I decided to become a Nurse after the birth of my second child, and became a Certified Doula first, convinced that I'd spend the rest of my life in L&D. When I went through A&P II, though, I fell in love with all things Cardiac. Then my first job ended up being in Surgery. You never know.
Enjoy the journey