1) Who are you planning to train?? Athletes? Fat loss candidates? Your neighbors?
Nowadays, there are so many people out there that do not have a knowledge of what a "certification" is let alone the 100s of different organizations that do it. If you are gearing towards the mass public then Yes you need some sort of simple cert (CPT) through one of the better known bodies like ACE, ISSA, NSCA (I'd go with that one)....because the general media is telling people that they need to look for trainers with those credentials, so you better have it if you are training the masses.
2) What's your background? If you are Louie Simmons, you don't need anything else but your name. Unless you have 20years of experience in a particular field (PL, throwing, etc...) and everyone knows you....then you really can't put yourself in their catagory.
Certs are only as good as what they can bring to the table. Having it is definitely advisable because that's what the general public is looking for. You would want a big name people recognize. After that if you want to get additional certs like for Kettlebells, Golf Biomechanics...or whatever...it will also add to your resume and carve your niche in whatever area of fitness you are interested in. Many are also just good extra educational tools for you to use with clients.
1) Get the NSCA-CPT or CSCS if going into sports team training (it's recognized everywhere....like Visa)
2) Have a niche for your training.....athletes, powerlifters, fat loss, mountainclimbers...whatever. Just don't be a "personal trainer"....those people are everywhere and most don't bring a good reputation to that title. Plus overtime that is how you become a Louie Simmons...getting known in a particular industry as the "expert"...at that point, people will come to you.
3) The credentials will get your foot in the door.....client testimonials will close the deal, so get those up on your site or where someone can see it.
4) and of course get liability insurance (like $200 a year)