Calculating pH of a Weak Acid

Could somebody please answer this question for me and show me your working so I can hopefully see how it's done? I'm fine with strong acids that completely ionise but I'm struggling with weak acids.

Calculate the pH of 0.01 mol.dm-3 propanoic acid (a weak acid, which partially ionises and has a dissociation constant of 1.34 x 10-5 mol.dm-3) at 298K.

Do you know the formula for the dissociation of a weak acid?

ka = [H+][A-]/[HA]

assume [HA] is constant