What should be the development planning tools before writing the code?

Question

.NET

After studying the C# language and writing a number of simple programs, I realized that for quality work and in order not to get confused myself, it is necessary to PLAN the application structure, its classes, methods and objects in advance. And also for correct and easy extensibility in the future.
Who uses some tools for these purposes? (not necessarily under C#)
And recommend a book for a beginner.
Thank you.

Answer 1

For .NET, there is NClass. A lightweight, no-frills class diagram tool. True, I used it mainly to generate diagrams from already compiled assemblies, and not for its intended purpose, so I can not vouch for the convenience. But it might be useful.
PS Itself, if the project is more or less complicated, I design it on a piece of paper. If something is simpler, then right in the code I make a "sketch" in the form of a set of interfaces (trying to take into account possible extension options, but without much fanaticism), and then I start implementing it. Something like this.