You have the best mentor already, The Internet. Keep yourself engaged with the trending Technologies and Repositories.
Best part is, pick a Github Repository->Issues Section, You'll find label like "Good for First Time Contribution" or something resembling. Pick one and start trying to fix the issue. Even if you couldn't fix the issue but you'll surely start to learn the standards and convetions of Production level code. And, Don't restrict yourself by calling "on the job learner".
Here is a Github Link having some projects good for beginners to contribute:
github.com/MunGell/awesome-for-beginners
Cheers!