Probability mass function

The distribution of a random variable X is given by

$\displaystyle F(x) = \left\{

\begin{array}{lr}

0 & \text {if} \hspace {3pt} x <-2\\

\frac{1}{2} & \text {if} \hspace {3pt} -2 \leq x <2\\

\frac{3}{5} & \text {if} \hspace {3pt} 2 \leq x <4\\

\frac{8}{9} & \text {if} \hspace {3pt} 4 \leq x <6\\

1 & \text {if} \hspace {3pt} x \geq 6\\

\end{array}

\right$

Determine the probability mass function of X.

I have no idea where to begin. Can anyone show the process to determine the probability mass function so that I can repeat it for other problems?