Most of the comments I have read in reply to your request entails good grades, and the right classes. Yes, these are important and if you are on that track then I applaud you. But knowing nothing about where you are at in your life may I also say that determination accounts for much of your future success. If I may share my story:
When I was in high school I was told (by a not very nice step-father) that it was ok that I was stupid (I wasn't I later found out) because I was a woman and the only thing I really needed to know was how to cook and please a man. When I was 22 I had a beautiful daughter born to me, she fell sick with spinal meningitis at 6 weeks and we thought she was going to die. I lived with her at the hospital for 10days in ICU and watched with great interest, the nurses that came and went. I caught on quickly with what they were doing to my daughter. The procedures, the antibiotics, the tubes, and machines -- I was able to comprehend their uses, the reasons for them and what was going on in my little girl's body.
I talked with the nurses that were caring for my little Rachel, and began to realize that I could be like them. I wanted to be like them and I needed to be like them. I trained to be a nursing assistant at a nursing home. From there I entered a community college to begin the path to my RN, at the same time I was working and raising two children.
I worked hard and took extra classes, and instead of continuing at the community college level I was able to transfer to the University. In 3 1/2 years I graduated with my RN and a BSN, with honors.
I share this with you because not every one of us does things the "right" way. I encourage you to get the best grades possible, to take all the pre-college exams that you possibly can, and listen to the right way to do it. But if that is not you, don't give up. Just do it another way. The main point is do it.
At this point my daughter is a healthy, happy 12year old. She knows this story by heart but vows never to be a nurse she wants to be a veteranarian instead :-)
Congratulations on knowing what you want to do at a young age, I wish I had. Good Luck!