OK, for starters, it helps if you don't switch between Leibnitz and Newtonian notation. Stick to one or the other. I tend to prefer Leibnitz.
Second, you need to understand that is an OPERATOR meaning "take the derivative with respect to x", while is a function, namely the derivative of y with respect to x. It can also be written as y'(x) if you prefer Newtonian notation.
The basic idea is that y is a function of x, so if you have a function of y, you can differentiate it with respect to x.
Now take the derivative of both sides again to find the second derivative.