None, None, None
No extra schooling is required to become an ER nurse, but experience helps get your foot in the door. I started volunteering for a fire department and was able to get my EMT -B for free through my volunteering. Another volunteer also worked at an ER and she was able to get me an interview at an ER. I worked and volunteered as an EMT through nursing school, which helped me get hired as an ER nurse after the NCLEX.
I feel working in healthcare is the best way to see if you will enjoy the work, gain skills, and make contacts. There are many entry level healthcare level positions and most require a license. These positions typically have flexible schedules which allow people to continue their schooling.