I have completed both of the following:

1) https://www.coursera.org/learn/learning-how-to-learn

2) https://www.coursera.org/learn/machine-learning

I highly recommend both of them, if they cover a subject you're interested in, though if you only have time for one, they are listed in descending priority order.

Given how low it's time+energy requirements are, and how large the pay-off has been, I recommend Learning How to Learn by Dr. Barbara Oakley to everyone regardless.