[pentesterlab]

As a dev, you should start learn about the most common bug classes and get a proper understanding on what is happening and how you can prevent them. Then it's worth looking into how you can exploit them. If you work for a big company, you may be able to spend some time working along your security team, that will help you get a foot in the door. Security teams are always looking for people who already understand the code bases and deployment processes and want to inject security everywhere :)

<shameless_plug>You may like https://pentesterlab.com/ if you are looking for a course</shameless_plug>