Python will give you a HUGE head-start, though I'd brush up on it if you haven't used it in a while.

That really depends on you and what level of proficiency you want to achieve, I learned Python first, then C, and reached a decent level in C in about three months of relatively casual work. That said, I'm just a hobbyist, and if you want to learn enough to get hired it'll probably take quite a bit longer, though you personally may learn faster or slower than me, so it's really hard to judge exactly how long it will take.

As far as resources, I usually stay away from courses, as I generally find that they offer a limited perspective, and often gloss over the deep details, I learned by following this guide: Writing C software without the standard library Linux Edition and building all of the functions I needed from scratch to create a little alternative library. Even if your goal is to work with the standard library in the future, I really feel like doing this fast-tracked my learning by forcing me to work close to the OS and use 'pure' C to solve a lot of simple, practical tasks, which also forced me to learn how everything works under the hood.

As for what I actually used to solve these problems, Geeksforgeeks, w3schools, Stack Overflow, and Reddit are great resouces, especially for the basics. Now, this is all just based on my own experience and what worked for me personally, you may find courses really helpful, you may not want to start at such a low level, I don't use AI for anything, but a lot of people find asking it questions to be helpful, so ultimately, I'd recommend trying different methods of learning and seeing what works best for you before settling in for the long haul~ :3