How to find the ionization constant of an unknown weak acid?
Look at the answer given here and modify it:
How to set up equation for buffer reaction?

In your case the concentration of base $c_B \approx 0$ and $[\ce{OH^-}] = K_\mathrm{w}/[\ce{H^+}] \approx 0$, so that the equation reduces to $$K_\mathrm{a} = \frac{[\ce{H^+}]^2}{c_\mathrm{a}-[\ce{H^+}]} \approx \frac{[\ce{H^+}]^2}{c_\mathrm{a}},$$ 
as $c_\mathrm{a}$ is so large.