Notes to a young trainer: how to get started as a personal trainer

I highly suggest you pick up a well-known general personal training cert like the NASM CPT. NASM is a very dynamic, and forward looking cert. Their info is very good and up-to-date.
Beyond that, I highly recommend you pick up a second non-traditional training cert in something you’d want to be your specialty. Something like kettlebells via me, Steve Maxwell, or the IKFF, would work nicely. Other good specialist certs are Monkey Bar Gym with Jon Hinds and Tactical Athlete with Jeff Martone. I recommend these guys because they are all ethical, no-nonsense guys that have great material.
You need a specialty cert because the CPT certs are usually good, but basic and largely theoretical. The CPT cert would be necessary to get a job. The specialty certs are mostly practical and will show you how to actually get good results with clients. Having both would put you ahead of other beginning trainers.
If you plan to just train people as a side business on your own, only the specialty cert is necessary.