Reading books. There are some great books on programming out there. I would strongly recommend a A Players Guide to C#. It’s structure, practice problems, and explainations of the basics were far better than any free guide to programming that I’ve seen online. There are a lot of other great books out there too.

It also doesn’t matter too much about a book being outdated when you are only studying the fundamentals.

A lot of the stuff you read online has never been fact checked or edited for clarity. Some of it is great, but most of it is not.

Have you used Java before? It’s not a big leap from Java to C#, most of the concepts are the same. At least C# is a memory safe language, although I’m not sure if you’re unfamiliar with the code base and languages if it really makes a difference which language you use here. Hopefully your coworkers can help hold your hand a bit here, this situation sucks.