You could probably go through a couple of blogs and videos online to get hold of basics.
Check out this series of blogs https://www.edureka.co/blog/what-is-selenium/
In case your more comfortable with video tutorials, have a look at https://www.youtube.com/watch?v=5FUdrBq-WFo
After your comfortable with the basics, try out a couple of hands-on projects that would help you gain practical expertise on the subject. It gives you an added advantage if you are certified in selenium.
For Selenium certification, check out https://www.edureka.co/selenium-certification-training
Hope this helps!