A strong acid is one that completely dissociates, although we must bear in mind that this statement would not be true in highly concentrated solutions. An example of a strong acid is hydrochloric acid.
$HCl +H_2O\rightarrow Cl^{-}+H_3O^+$
Note that I have used an arrow to indicate the full displacement of the equilibrium to the right.
With this type of acid, the autoionization of water is negligible and the only source of protons is the acid (valid for solutions that are not extremely dilute).
From the above it follows that: $[HCl]=[H_3O^+]$, where $pH=-log[H_3O^+]$
Let's see an example: Calculate the pH of a 0.1 M HCl solution.
Being a strong acid $[HCl]=[H_3O^+]=0.1\;M$
pH=-log(0.1)=1