How do I reduce radian fractions? For example, I need to know $\sin (19π/12)$. 
I need to use the subtraction formula. How do I get $(\text{what}) - (\text{what}) = 19π/12$? I am stuck at what are the radians
Do I divde it by something? What is the process? 

The denominator is $3\cdot4$ and you (should) know the values for denominators $3$ and $4$, so just decompose in two fractions
$$\frac{4\pi}3+\frac\pi4.$$
Now apply the addition formula.

This can be obtained by solving
$$\frac a3+\frac b4=\frac{19}{12}$$ or
$$4a+3b=19.$$
You can work this out by trial and error.
