Nitric acid (#HNO_3#) is a strong acid (we refer to it as a mineral acid as opposed to carbon-containing organic acids which are mostly weak). This means that we assume it dissociates completely in solution. Therefore, since is it also monoprotic (it provides one mole of hydrogen ions per mole of acid), the concentration of hydrogen ions in solution equals the concentration of the acid i.e. 0.125M.
Now we simply use the equation that defines pH, thus:
pH = -lg#[H^+]# = -lg0.125 = 0.903 (to 3s.f.)
NB 'lg' denotes logarithm to base 10.