Have you looked on udemy.com ? I learnt 60% of my coding in php, html, c++ and c# on there. I learnt there more than I did at university.. lol
They way I learnt is by learning php first, procedural then oop.. It's quite easy to learn and understand... Then C# because it's just so powerful and universal and we had this at university, once you understand well enough c#, coding anything in unity3d is a breeze as long as you look up stuff on documentation. You will be able to write your own plugins, extend plugins, and write new classes and stuff.