To pursue a commercial certificate under Part 61, you will need to already hold a private pilot certificate, and you should hold an instrument rating. You can technically earn the commercial without the instrument rating, but doing so puts limitations on your certificate that make it far less useful, so almost everyone gets the instrument first. That is why the standard path is private, then instrument, then commercial.
You will need to be at least 18 years old, be able to read, speak, write, and understand English, and hold at least a second class medical certificate, since that is what commercial privileges require. From there, it is a written knowledge test and a practical test with an FAA examiner, same as the ratings before it.