I guess it depends on what technologies you want to start using. I would suggest if you are currently working for a company which implements automation testing, start with those technologies. If they are flexible someone might show you around their framework or even let you check in a few simple tests to begin with while you are learning, if you have a friend there they could review your code.
I’m currently reading Java for Testers by Alan Richardson, which is a good place to get started. From the book:
“This book is aimed at people who are approaching Java for the first time, specifically with a view to adding automation to their test approach. I do not cover automation tools…
Testers use Java differently.
Most Java books start with a ‘main’ class and show how to compile code and write simple applications from the command line, then build up into more Java constructs and GUI applications. When I write Java, I rarely compile it to a standalone application, I spend a lot of time in the IDE, writing and running small checks and refactoring to abstraction layers.”