I can't remember where I learned the basics, but if you aren't new to programming in general then I think there are only three important things you need to know

I can't remember where I learned the basics, but if you aren't new to programming in general then I think there are only three important things you need to know: what is a pointer, what is a macro, and always free what you malloced. Reading a book or tutorial will be helpful for this. In my day it was K&R, no doubt there are more up-to-date ones now. But the next step, which I think is far more important, is to look at the source code of tools that you actually use in real life. Things like cp, or wc, or head. You've probably used them for years without thinking about it, but they're all written in C. Don't look at the modern GNU versions just yet, since they can be packed full of complex functionality, start with micro implementations like Busybox or Toybox. Then look at some OSes that are known for super clean code like NetBSD or OpenBSD. In those OSes you have the benefit of being able to go back to the version that existed 25+ years ago, so you can read through the diffs to see how they adopted new features and found new ways to address C's biggest flaw/risk - memory exploits. If you're interested in kernel programming, check the Linux and BSD sources, and lurk on the mailing lists for a while to see how people talk about the code. The review process tends to be a bit more brusque than you might be used to on Github, but it's often detailed and results in code that is of a relatively high quality, or at least a consistent standard. It's a great place to learn.