The pH of a 0.1 M solution of HCl is 1. Hydrochloric acid is a strong acid, so it dissociates essentially completely in water into hydronium ions and chloride ions. A 0.1 M HCl solution therefore has approximately 0.1 M hydrogen-ion concentration, and pH = -log10(0.1) = 1.
The pH scale is logarithmic: a solution at pH 1 has ten times the hydrogen-ion concentration of one at pH 2 and one-tenth that of one at pH 0. The calculation assumes a dilute aqueous solution and uses concentration as an approximation for hydrogen-ion activity.
A common mix-up is confusing pH with pOH. At about 25 °C, a pH of 1 corresponds to a pOH near 13. Very concentrated HCl solutions require activity corrections, while extremely dilute solutions require accounting for water’s own ionization. Neither complication changes the standard classroom answer here.