Welcome to AN
I had the same dream (although it didn't hit me until my senior year in nursing school
). There are a lot of posts regarding what you're asking in the NICU forum here on allnurses and I highly recommend utilizing the search function.
The short of it, though, is that you go to nursing school (preferably going the bachelor's route or if you're going the community college route, getting your bachelor's soon afterwards) and then after you graduate, you take the NCLEX, which is the national exam that all nurses must take in order to practice.
Then you apply to various hospitals to become a NICU RN. I recommend going to an academic center (what they call a "teaching hospital") where new grad nurses are plentiful with generally excellent orientations, IMO, for NICU nurses. You may not get into NICU right away, but there are various strategies you can employ to increase your chances (again, use the search function). I applied to NICUs about 6 months before I graduated and received my job offer and worked in my dream job for the past 5 years.
There are various certifications you can earn, but most of them require that you be a NICU nurse for awhile before you can be certified. Notable exceptions include the Neonatal Resuscitative Program (NRP) and S.T.A.B.L.E. program. I took NRP as a nursing student and it was doable, although very hard since nursing school is generally geared towards adults and I had no background with infants.
Once you've gained some experience (and two years is the minimum, although I think that unless you're at a high acuity NICU, you really need more), you can apply for NNP programs. There is a thread in the NICU forum that lists all the NNP programs in the country, although it hasn't been updated for a couple of years. Once you complete your NNP program, you are eligible to take the boards for your NNP license and then you can start practicing.
I've loved most minutes of my time in NICU and school, while rough at times, is amazing. It's completely worth it.
Wishing you the best and hope you'll keep us updated on your progress!