I'm not an ER nurse, but in nursing school and hoping to get a position in the ER when I graduate in May.
While you are in high school, keep your grades up and take a science course each year/semester.. I know my school counted what you took in high school as part of satisfying the requirements for acceptance. If you didn't take what they need then you have to take those courses before you can apply into the nursing program.
Becoming an ER nurse is not much different than any other specialty to start with- you have to become a registered nurse first. For that you need to look into what kind of program you want. Like some of the others said- talk to your guidance counselor. There are Associate Degree RNs (ASN or ADN are the acronyms you will see alot) and Bachelors Degree RNs. (BSN) Both are a RN, take the same state board exam after graduating with their degree, and the most simple way to state the difference is that BSN is sometimes required to go into management or highly specialized areas in major metropolitan areas (big cities). For example: If I wanted to work in a downtown Chicago hospital in, say, the NICU (neonatal intensive care unit... where premature babies and sick newborns go) I'd most likely need a Bachelors. But, to work on a regular med/surg (medical/surgical recovery) floor I probably wouldn't. In most local hospitals a BSN is not usually required to work as a floor nurse in any specialty (specialty means other than med surg, like ER, ICU, Telemetry, CCU, OR, Peds etc) .
That said, there are exceptions and I highly generalized and simplified it.
So basically first you need to decide if you want to go for your associates (received from a community college- 2 year degree) or Bachelors (from a university- 4 year degree) Keep in mind many university have RN to BSN programs where you can continue on after you are licensed as a RN to get your bachelors.
After that you need to pick a school. Find out what their pre requisites are (courses you need to take to be able to apply into the program, and other general education classes you can take during the program), its a good idea to take those first. That way when you start taking nursing classes thats all you take.
Take any tests you need to take now. Go ahead and take your SAT and ACT when its time, that way regardless of what you decide you will be set as far as required testing goes.
After you finish your pre requisites you can apply into the program. Then you must complete the program, graduate, and take the NCLEX-RN exam... that is the board exam to get licensed to work as a RN. During your last semester of nursing school you usually start applying for positions. Apply for an ER position. After you get out of school and start your job, you will go through a (hopefully) lengthy training period to help transition you from a student to a nurse, and this will provide training in your particular position.
I think your best bet is to go talk to your guidance counselor. Also check out the student nurse forums here for an idea of what nursing school is all about.