This is perhaps not the best question to ask on SO as it’s bound to get opinions rather than answers, with this in mind, I will give you my opinionated answer.
As your first real life application, it’s probably best you go with something you’re somewhat familiar with, either that or find a solution with simple, quick set up guides for beginners.
Both of the languages and environments you mentioned have these.
Myself, I would use C#. It’s always been my go to language to demonstrate some real world problems as it’s generally quite quick to get going. Once you download Visual Studio and start writing code, it all just comes together. There is plenty of material available at MSDN which has examples of code with how it should be used, there are even some tutorials on there, I believe one of them is actually a book shop tool.
Have a look there and see what you find.
Ultimately, you’ll likely have to make a decision as to the language and development style / environment that you like, but for now, you have one biased opinion from a happy and content .NET developer.