First of all, congratulations on your acceptance! Now, find a way to do as little as humanly possible for the next 9 months. The next 7-10 years will be busy beyond your belief. My advice:
1 - Work your tail off in med school. Learn as much as you can, then learn some more. That foundation of medical knowledge is going to help you in any field you go into (except maybe ortho, it's just bones). The harder you study in med school, the more doors will be open for you when you decide what to do.
2 - If your school has a home EM program, reach out to the program leadership or individual attendings and express your interest. If your school doesn't have a home program, reach out to programs nearby. They understand the spot you're in, and if they don't just move onto the next one. Find out if you can shadow in the department to gain some clinical exposure and make contacts. Find ways to be helpful when you're there.
3 - Be "that girl/guy" for your clinical teams (but not "that girl/guy") on clinical rotations. Understand that clinical medicine is about learning to apply that foundational medical knowledge in a real world environment. Help your team out: be early for signout each day, get the cup of water the patient asked for, figure out where the translator phone is kept, take the paperwork to the fax machine or the person who lords over it. Do not leave early, do not hide, do not lie, and do not worry so much when you can't differentiate hypovolemic hyponatremia from euvolemic hyponatremia (we don't care, and even IM is going to consult renal). We do everyone's job in the ED, so every rotation is educational.
4 - Don't fail Step exams, repeat years, have ethical, behavioral or professional issues, etc. These will hamstring your application to any specialty.
5 - 4th year EM rotations and your SLOEs (Student Letter Of Evaluation - I think??) will be the most critical part of your application. Then clinical grades, comments, step scores, the usual. Nobody really cares about research or volunteering unless (maybe) you do it in the EM program you match to.
Best!