Nowadays, there are so many certifications to choose from. From ACE, to NSCA, to NASM, it’s hard to know which one is the best to go with. The easiest way I can recommend breaking it down, is knowing which general direction you are most interested in going. A trainer that eventually wants to work in a professional sports setting is going to need a different certification than a trainer who just wants to train the general population.
Some certifications require more prerequisites than others, and some require a college degree. Here are some rough guidelines.
ACE - ACE Certification is one of the easiest certs to get. It doesn’t require a college degree, and it’s not too expensive. I recommend this one if you are tight on money, and just want to get your feet wet working at a box gym (although check to see what their requirements are, as some gyms require specific certifications), or just want to be confident training some people privately on the side. The downside to this one is, some gyms (usually the higher tier ones) won’t recognize this cert as a valid one for training people, due to the fact that it doesn’t go as in depth as some of the other ones.
NSCA-CSCS (Certified Strength and Conditioning Specialist) - NSCA has regular personal training certs as well, but this one is geared towards training collegiate and professional athletes. If your goal is to work with athletes at those levels, pretty much all of them are going to require you to have this cert. Keep in mind, in order to obtain this, you need a bachelor's degree (doesn’t have to be in Kinesiology).
Also, if your goal is work on the professional level, you are most likely going to have to start by interning at a college, or for a farm team. Some of these will pay you, and most won’t. That part can really suck, but the huge advantage is a lot of these internships can lead to a job within that team's coaching staff, or referred to one where the head coach knows other coaches. Their network is really small, so making sure all mentors you encounter are fans of you is a must.
NASM - NASM seems to be the most popular certification in most chain gyms. It’s a very reputable brand and the one I’ve found most gyms ask for when applying. They also have additional certifications geared towards elderly populations, chronic injuries, etc.
NCI or Precision Nutrition Certification - If you’re looking to get more into nutrition coaching, or want to further round out your skills as a trainer after getting your personal training cert, these are two highly recommended nutrition focused certifications.
I think EVERY trainer should have both a personal training certification AND a nutrition coaching certification. If you TRULY want to work, and help people get better on ANY level (general population or athletes), while I don’t expect you to have a full nutritionists scope of knowledge, you should know how eating plays into their life. The perfect training program is useless for someone's goals if they aren’t fueling it with the right foods in their body.
More importantly, it’s not just knowing how to program for someone. Being a trainer more than anything means being a psychologist. Let’s be real. Everyone knows they should be working out more, eating better, and listening to you. But many times you’ll find they still don’t. Why? Because they have emotional attachments to certain behaviors whether it's towards eating, or adherence to a program. The best trainers in the game, know how to address these issues, and create behavior changes that help their clients overcome these challenges and teach them how to make long term changes.