What to study for the opportunities to earn as a freelancer?

gurus!
I am interested in gaining some new skills. I want to learn something that can earn me a good sum of money online. And I don't want to be a part of the earning schemes that "do not require any skills or prior knowledge".
Now, should I learn programming? If yes, what kind of programming? Or should I be a graphic designer? Should I do something else? Kindly keep these points in mind while suggesting the field for me:

1. It doesn't matter if it's hard or easy. I'll learn.
2. It should be something that I can do online or sell online.
3. It would be great if I can learn it online, for free.

I am thankful for your advice in advance.
Love!