Talk to your high school counsler to find out about nursing program in your areas. Graduate from HS and start applying to colleges with nursing programs
. There are several different types of programs, ADN, BSN, diploma. If you need help deciding which one is right for you - there is a board here on allnurses that discusses that very topic.
While waiting on acceptance to the nursing program (usually pretty competitive) you can work on your pre-reqs or classes that are required prior to actually starting the nursing program. The entire process can take anywhere from 3-4 years, depending on the program and/or degree you obtain.
Once you finish your nursing program, you'll be eligible for the NCLEX-RN which is a national licensure exam. Once you pass the NCLEX, you are then an RN.
Allnurses has great boards that are devoted to pre-nursing, nursing school, NCLEX, etc....that can be of great help to you as well during your journey.
As far as how to become an ER nurse, you apply once you graduate and you become an RN. Your actual training is all done on the job (OTJ) and most good ER's with good solid orientation programs will provide you with all the classes/certifications you need your first year or two.
However, while in nursing school (NS) you will be exposed to many different areas and many different view points on entering different fields of nursing. You may very well change your mind while in school and decide a different path other than ER. That's the great thing about nursing - there are MANY paths a person can take.
Good luck in your journey!!!