Hey Basi -
First & foremost, good to hear that you have the Tox Bug... I caught mine early on in med school (pharmacology is just so damned awesome)...
As for fellowships, to be frank, don't put the cart before the horse. Post-residency fellowships in medical toxicology don't care much about the MD/DO thing (in fact, my fellowship director was a DO, as was one of my junior fellows)... as such, be a rock star in med school & on the COMLEX/USMLE so you can get the residency you want. This will result in you being a happier & more productive resident, which, when you account for LoR's at the fellowship-applying level, will have your PD much more enthusiastic about recommending you for our field.
If you're very interested, then research in the field isn't a bad idea; especially when you have some time in medical school (less research time in residency), and can use it to "double dip" = residency interviews AND fellowship interviews.
Rebuilder's right, though, MT is a relatively small field with the number of slots/year for fellowship; and MT is sponsored not just by ABEM, but Peds & OccMed - though the majority of fellows come from EM. That being said, it attracts a relatively small pool of applicants (at least now), so getting a spot is definitely doable provided you're somewhat flexible. More programs are popping up every so often, which opens up new opportunities in geographically distinct areas of the country.
As for the application process, currently it's an "outside the match" process - you typically interview with the programs you're interested in, and if you're offered a position you take or leave it. There's talk, however, about trying to enter a match process within the next few years, so this may change.
After one accounts for marrying my wife, my fellowship in MedTox was the best decision I've ever made - I love my job, I have my niche, and I have a unified body of knowledge that many in the hospital respect & appreciate.
Cheers!
-t
