While I was in nursing school I became a nurse assistant at my local ER. Once I became a nurse the would not hire a brand new nurse. Therefore I had to work on my tele floor. I stayed on tele to get my experience for almost 3 years. They would've taken me back in the ER sooner but I stayed on tele. I am now back in the ER as an RN. Since you work at the hospital already that's a great step. Try to discuss your wants with the ER manager. To prepare yourself for the next step do your research on oxygenation, normal vitals, and CPR. If you want to be a critical care nurse you need to just take a leap and talk to the manager about getting some sort of role in the ER to get your foot in the department.