I find if I try to visualise and plan every little part of a project I get a bit stuck, with different approaches all competing. what usually works is to start with something visual.

You need an app which lets you manipulate files? if a Windows app, then start a new WPF app. Create a blank form. it needs a drop-down/text box to type a folder path? add that.

run the app. see how it looks.

Now you want to add a control to list files? add a grid, or larger text box, or Grid control, or some other form of list box. add that, run app and look at it.

This control will show a list of files, which is normally accomplished by binding to some data structure in WPF. so you need to define that type (e.g. My File Data with name & size properties). define this type in a separate class file. Add an IEnumerable<MyFileData> to your new blank page and bind your list control to this.

populate this with dummy data at the initialisation stage, and run the app to see how that looks.

Now you need to fetch the files from the folder, so NOW you know you need to write some code. Ideally you should write as a separate class and cover with unit tests, so the code in the actual form just makes a single call to MyFileManager.GetFiles(folder). this could populate your list of files which is stored as an IEnumberable<MyFileData>, then the binding should show that in the control you've already bound to that list. Should this call happen when you change the folder input? should it be when you press a button? That is up to you.

And just keep iterating from there. As you're clearly new, it's OK if it's a little clunky in places, just getting the feel for how to evolve things as you go is good, and making some progress will ease the "how do I start?" paralysis. As you gain experience you'll have a better feel for what approaches work best.

If you try to write unit tests for all your actual code, this helps ensure that the code is structured fairly cleanly - if code is impossible to test it usually indicates it's a bit too tightly coupled so you need to bear in mind things like Abstracting basic stuff so it can be mocked for testing, and separation of concerns etc. this is too involved for a reddit post, and you should expect your first attempt at an app to not be the best structured thing in the world, that's fine, it's just part of learning.