There are some other threads on here that go into great detail about it, but the general concensus is, you first need to know if you plan to train people in another person's gym or if you'll be doing it in your own studio or other people's homes. Some gyms require specific certifications or sometimes even degrees before they will let you train people there, some are less specific. But, the last thing you want to do is get an easy certification only to find that none of the gyms in your area will let you train there for not having met their requirements.
Start with the plan of where you'd like to train, if it's at other gyms, ask around and see what their required certifications are for employment consideration. ACE is pretty much the standard entry-level certification that doesn't really get you a great qualification (many places in my area won't accept it from trainers), but opens the door at some facilities and will let you train people independantly of other gyms if you plan to do it at your own location or someone else's home. Once you know where you plan to start, you can plot the best course of action.
Also, don't forget that you'll need lots and lots of liability insurance. I hear that $3 million is becoming the norm for what trainers are expected to carry at some places now, so that's something to factor in as well and ask about at gyms in your area as well.
Lots to think about!
Thanks to the OP for starting this thread and for this response! I've been seriously considering getting my certification but haven't had as much time to look into going about it the best way since starting my new job.