In order to become a critical care nurse you must earn either an associate's degree or a bachelor's degree in nursing science then sit for your state board test and pass it. This is a general knowledge test. Passing all of the required classes requires commitment and effort every single day. Personally I don't consider myself super intelligent, but I was able to excel in my nursing program including my master's degree by applying myself every single day to the goal of earning those degrees and my nursing license. I can guarantee you that you will never, ever regret earning your nursing license. It has been an incredibly rewarding career for me as well as providing me and my family financial stability.
Life is a series of choices. You can choose to go out at night or you can choose to stay home and study. You can choose to watch television or to do your readings. This doesn't mean you're not having fun, you will be. But fun will be second to studying not the other way around.
If this sounds harsh I want you to think about this: when you become a nurse or other people become physicians you have the lives of other people in your hands. Wouldn't you want to be taken care of by someone who took their studies seriously?
Studying to become a nurse involves classes in chemistry, biology, microbiology, math, pharmacology, anatomy and physiology. When you have completed your degree will have a generalized understanding of body systems, the nursing process, abnormal psychology, ethics, and disease states. You'll take your state board and pass it easily. At that point you can enter the nursing field and hopefully find a hospital that has need for critical care nurses and has a training program.
I hope you find this answer helpful and best of luck to you as you pursue your dream.