How can I start in selenium automation testing?

Selenium Webdriver from my experience is a plugin available for download from NuGet Package manager in Visual Studio. It's purpose is to mainly automate the manual tests that are done, using scripts in any supported language. We might additionally need the respective browser drivers as well to open the browser automatically using scripts and enable automation. 

The question is very broad to begin with. Selenium webdriver is a tool that can be used to establish a manageable automated testing solution. You need to establish quite a few things based on your project/product environment, load, technology, development model, database etc. inorder to fully implement a selenium based solution. You need to develop an framework suitable for the product, test data, environment, execution model, select test cases to be automated, get the RTI after implementation etc. 

Selenium is an Open Source tool, anyone can download it from the internet and install on his/her local machine and start writing test scripts using any language like java, c#, Python.