Learning from scratch is something I thought could easily be done as I've always one things from scratch but it caught me off guard during my full stack journey.
I would totally recommend HTML, CSS, JavaScript, they all now fall into almost on category, getting as much done is most important on frontend. Also looking at other Frontend Frameworks such as React or Angular JS, again another starting point.
However when its backend, you must take almost an utter different approach to writing code as well as the logic behind. I refer to it as almost writing backwards (god only knows why). As well as you would also need to learn business and application logic, so e.g if you were building a full stack application then you would need to be varied in the knowledge and fully open to learning something new.
I've spread myself quite thing but it is possible, but you just have to hop between languages and know right '' might work in this language but it won't work in the same way as another langauage.
I came across a good learning application recently called "Programming Hub", for once a semi decent android/ios application you can learn and read about a specific language from basics to advanced and totally would recommend it.
Backend I would recommend Node JS + Python, they can do hand in hand as they are the most popular and also taking over PHP / MySQL. Once you got Python down you might find its easier to then look at JavaScript or another framework. Would also suggest learning about the difference of the "top down" approach and "bottoms up" approach as its two building models should we say in a way. It makes the difference when you're coding between html and then javascript/python. I'm a junior full stack at the moment i'm working with python so if you ever need a hand or help just give us a shout x