Topic. 
Start here. Read all the lessons, write all of the code. No copy-pasting.
Think of a simple game or application you want to make (like Pong, Tetris, top-down shooter, etc.) and plan how you think it should work.
Use the Unity Scripting Reference and MSDN to help you. Ask questions on this forum when you run into weird problems you simply can’t solve alone.
You should write code as often as possible. Once you have a small game made, write scripts to make the game bigger, more challenging, more complex, more fun, etc.
Other useful resources are C# Corner and Stack Overflow.
And please use a search engine to find information before asking in the forums or the Answers pages. Often times, especially when starting out, there are many questions already answered floating around.
Good luck!