Archimedes gave some great advice, but I'd add to call around to the gyms in your area and find out which certifications they prefer. In my area, every single one said that ACE or NASM were among their top choices, so I went with NASM.
I looked into working for a local chain gym, and decided to strike out on my own instead. It's been an uphill battle because I have to keep my full time job for the $$ and for the health insurance, at least for now. I live in an area hit pretty hard by the economy--most folks don't have the disposable income to hire a trainer. I get a LOT of requests for free advice, though
If you're self-employed, you do need liability insurance; a good policy will cost you a couple of hundred bucks a year. You'll also need to stay on the lookout for CEU opportunities, as most certifications require you to renew every couple of years and keep your skills up to date.
It's been tough, but I enjoy every minute of it, from planning workouts to researching various exercise alternatives to suit different clients, to checking out new gadgets and equipment, to looking back with them at their progress and planning for their future goals. I did look into getting a yoga certification at one point, but it seemed ridiculously expensive, so I shelved that idea, at least for the time being.
Go for it!!