Five languages
The five languages I am focused on
Resources
A good book can provide a well-organized introduction to a language. Online material can be a good supplement once you feel comfortable with the basics.
Practice
Alongside programming exercises, working on a new useful project can be helpful when learning a new language. Usually you return to it regularly even if there might be quiet periods. The more projects in one language, the more opportunities to work with in the future.
Exercises
The C Programming Language (2nd edition) provides one or more exercises in each chapter. Those exercises can help practice what you've read and catch what you might have missed.
There are websites and resources dedicated to programming exercises (eg codewars.com). Practicing them regularly can help with learning a language, and it can be easier than starting a new project.