what agency you want to be certified by, depends on where you want to dive.
the single most accepted agency all over the planet is PADI.
here in the USA you could go with any.
which is better? PADI vs NAUI vs SSI vs SDI and so on depends in my opinion only on the instructor who teaches the class.
it helps to know someone who comes highly recommended by others.
scubaboard.com is a place where you can find info on local instructors, just ask who they recommend and why.
don't fall into the trap of an LDS trying to sell equipment.
most LDS give out bargain prices in certifications, just to make up in equipment sales.
only buy your mask and rent all other stuff and find out if SCUBA is for you and if you want to stick with it, then do some research about equipment BEFORE you buy it.
how far you take your diving is up to you and depend of how often you dive and the need to replace/upgrade your equipment. when you decide to buy equipment, start out with a back plate and wing.
SCUBA is a very fulfilling hobby and you can go so many different routes that there is a new way to discover SCUBA all over again and again.
I started it 20 years ago, did my divemaster in 92 and did a lot of crazy things that I keep better to myself.
I found interest in tec diving and dive mainly doubles these days.
I wish I would have known someone who could have saved me a lot of money by telling me what the right equipment to buy is.
since I have started my saltwater aquarium I now find some interest in collecting corals and fishes for it.
as I said, there is always another route you can take when it comes to SCUBA.