Firstly, anyone meeting the requirement for a DevOps engineer can become a DevOps engineer. One needs to fulfill the requirement like having a basic knowledge about Windows & Linux. Most of us will have a pretty good idea about Windows & Linux, not so much. Moreover, most of the tool that you will be working on will be configured in Linux.
Then there are some tools that a DevOps engineer must know. One must know some of the scripting languages like Shell Scripting or Python. One must have knowledge about Git/GitHub, Ant, Maven, Gradle, SonarQube, Jenkins, Bamboo, etc. These tools cover VCS, SCM, build tools, quality analysis tools for codes, Integration tools, etc.
Furthermore, for External Artifactory Repositories, tools like Nexus & JFrog can be helpful & for Application Services tools like Tomcat, WildFly, etc. can be helpful.
This is just a small range of tools that will help you to make your career in DevOps, in actuals there are lots of tools that you will come across & have to learn in the go.