You can start off by understanding what is DevOps, how is it different from other approaches. You can then move ahead by understanding the different tools using in the DevOps approach. Get in-depth knowledge of each tool. Get some hands-on experience with the tools because just theoretical knowledge will get you nowhere.
There are many resources online - Blogs, youtube videos, books, these are a lot of courses for DevOps which trains you in a very systematic approach.