Selenium is the most popular testing tool since it is an open-source, portable software testing framework for web applications. Even though Selenium comes with a test domain-specific language (Selenese), you can use other programming languages such as Java, C#, Ruby, Python, etc. to script tests.
I would recommend this Selenium Certification Course by Intellipaat to start a career as Selenium Tester.
Also, watch this video on the career scope of Selenium Testing: