Since you already have some programming experience in a language similar to C#, I’d start with learning the basics of C# which shouldn’t take you long at all. Probably just skimming through any random C# tutorial would be enough. (High level C# skills are certainly helpful, but not actually required for Unity. You can create a lot just with C# basics) Then look through the Unity manual just trying to pick up the highlights of what the engine and Editor can do for you, and the secret sauce of how Unity works. A lot of this is around scenes and the scene hierarchy, GameObjects, components, and the Assets folder, and how they all relate to each other. Also make sure you have a grasp of what references are.
After that you can walk through some beginner tutorials or just jump into your own very simple project and learn as you go along.
As for your questions:
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Good beginner projects are building game prototypes around simple game mechanics. Look at the old Atari 2600 games for inspiration. Keep your scope extremely small, as however long you think it will take you’ll find out it takes much longer. Don’t worry about polishing the game to a publish ready state, just focus on getting a basic game that is fun actually working. Do not do a multiplayer game.
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Once you have a specific topic you want to learn, you can usually just google for that topic and find YouTube videos or forum threads on the topic.
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It is hard to give you sources when I don’t know what specific topics you want to learn about. Usually they are created by people who want to share something they have learned, or are in response to a question, and aren’t in a central location. Though this forum is a good source of info, and you’ll learn a lot just coming here often and opening a lot of threads where you can read other people’s responses. Often a year later when you run into a similar issue or need to work on a similar topic you’ll remember you once read about it and either remember what it said or at least know what to search for to find it again. There’s several good YouTube channels dedicated to Unity development. I think Brackeys is the most famous.
- I wrote a thread about it a while back. Tips for new Unity users