There are 3 ways to become a dog groomer.
Online courses are a waist of your money and time. There is no way to get the skills needed to groom dogs by watching a video. There is always someone willing to take your money and give you a certificate to allow you to call yourself a dog groomer. But if you want to go to an interview with confidence, and be able to make a great salary, you need to learn by attending an independent grooming school.
Get a job in a grooming shop and learn on the job.
This is a good start. Getting a job in a grooming shop as a bather or front desk person will give you exposure to the field of dog grooming. The problem is, very few places have the time to teach you to groom. Most places will keep you in the position they hire you in and not advance you to the groomer position.
Go to an independent grooming school such as Long Island Grooming Academy
When you attend a grooming school, you get individual attention and instruction. There are dozens of dogs to learn on and every breed that you will need to know when you go for that first grooming job. Your employer will hire you with confidence because they see you invested in your career and have the skills necessary to start as a groomer.