How do I calculate the degree of dissociation in equilibrium?
Let us first define the terms needed here.   


Degree of dissociation (DOD)
Degree of dissociation is the fraction of a mole of the reactant that underwent dissociation. It is represented by $\alpha$.
$$\alpha = \frac{\text{amount of substance of the reactant dissociated}}{\text{amount of substance of the reactant present initially}}$$  
Number of moles dissociated
It is defined as the product of the initial concentration of the reactant and the degree of dissociation


Now suppose you have a reaction like this
$$\ce{A-&gt;B + C}$$

The initial state of A is always the concentration of A (should be given in the question) while initial moles of B and C are zero (if anything else is not specified).
The final state of A is always defined as $\text{(amount of substance initially present) - (amount of substance dissociated)}$ while for B and C it is just $\text{amount of substance of A dissociated)}$.

Writing our equation again,
\begin{array}{lcccc}
 &amp; \ce{A &amp;-&gt; &amp;B &amp;+ &amp;C}\\
\text{Initial amount of substance}&amp; a &amp;&amp; 0 &amp;&amp; 0\\
\text{Final amount of substance}&amp;   a - a\cdot(\mathrm{DOD}) &amp;&amp; a\cdot(\mathrm{DOD}) &amp;&amp; a\cdot(\mathrm{DOD})\\
\end{array}