For free:
When you have time, it's probably a good idea to sketch and get used to teaching your hand to coordinate with your mind's eye. It doesn't have to be manga style, just to learn how to do things basically. But also, I think copying (for your own use, not to share around online) the manga/anime you like, along with learning a bit about breaking down the body and face into shapes to build upon.
I also second Proko! The videos are helpful and inspirational! I just used one to work on the composition of a cover I'm working on
I like Megamaster's tutorials, because he has chill music:
Not Free:
It's not "manga" style, but you can still learn a lot from Loomis and other drawing books. Mark Crilley's "Mastering Manga" series is fantastic: I love his instructions and how much variety there is.