[Login to see the link] - PowerBuilder is a RAD (Rapid Application Development) tool. Sort of like a development language that is purpose-built with lots of extra features to expedite the time to write apps (typically business-database oriented).
The reason I mention this, is because C# and "web API" have minimal overlap or in-common with PowerBuilder. For that reason, you're asking a question that's a bit like "How do I build a skyscraper?" It's not quite so simple as a set of resources to learn; there are fundamentals that require a minimum of one year just to get up-to-speed. After that, you'll have a great deal more time, specifically if you want to learn C#, which is far more diverse and cannot be learned properly, without truly knowing those fundamentals.
Imagine you wanted to become a professional in a sport. Say surfing. Imagine you just wanted to go straight from knowing nothing -- to being a pro surfer. No course can teach you that. Instead, fist you really need to get super comfortable with the fundamentals.
The reason this is so crucial, both C# and PowerBuilder, emphasize OO principles. And PowerBuilder is based around Relational Databases/SQL.
None of these things are insurmountable to learn; however -- they all require years of time, which can only be expedited by learning the fundamentals and really becoming an expert with those fundamentals -- before you move any further.
I'm not guessing -- I am a professional software developer who has done this many years. 22 years ago I build a major system in PowerBuilder for one of the largest banks in the U.S.
Fortunately we have a lot of great books on fundamentals, right here on SoftArchive. Try these out -- BEFORE -- you go any further and start learning PowerBuilder or C#.
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And a separate one just for SQL:
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These are all of the easiest things to get started with. If you do not have any experience with writing code, my next step would be learning some basics about Python or if you want something similar, but closer to C#, then learn VB.NET. Both Python and VB are the easiest languages that have access to many libraries to help you build applications rapidly (similar to PowerBuilder).
You don't have to spend much time in Python or VB, but just some very simple basics will help you understand things, making the process of learning any other language much faster and more successful :-).
Good luck to you!