Most TV critics are usually part of the team of journalists who work for the publication in which the TV column appears. Some publications use freelance journalists to supply the TV column but, when a vacancy occurs, they would normally offer the job to someone who's regularly been providing them with news and reviews (e.g. of local theatre groups) over a number of years.
you could start by sending some reviews to your local paper for the tv pages - if they like them and print them you will have evidence of your work and can use it to apply for other jobs