I am wondering how I can convert the function $\theta'' + k \sin(\theta) = 0$ 
into a system of first order ODEs?
I understand that I should set for example:

$y\_2' = \theta''$  

$y\_2 = y\_1' = \theta'$  

$y\_1 = \theta $

And then generally it is easy to set up the $(y\_2', y\_1')$ matrix. However I am wondering how I can do it as the variable is a function of sine?