https://www.washingtonian.com/2012/07/09/25-things-to-know-before-you-become-a-restaurant-critic/ http://www.wikihow.com/Become-a-Food-Critic http://work.chron.com/become-freelance-restaurant-critic-16205.html http://www.artofmanliness.com/2012/06/29/so-you-want-my-job-restaurant-critic/
I've attached some articles that I hope will help, but in general to be a critic, you have to be a good writer, and write in a way that brings a unique perspective to your audience. Maybe you're really funny, or talk about food or the restaurant ambience in a unique or different way, or only review food sold in trucks, or something like that. With instagram and other social media, you can start to gain followers to your explorations and food critiques by posting beautiful food photography and writing unique reviews in a way that will grab peoples attention. Maybe you focus on one type or food, or one area of your city, but beginning to establish yourself as a tastemaker can be important. Getting any kind of journalism or writing experience, and especially as a critic or reviewer of something (even books or movies!) can help you gain valuable experience and to define and refine your voice as a writer.
Good luck and happy eating!