For a first order reaction, a graph is plotted between log [R]_t on $y$ - axis vs time (minutes) on  $x$ - axis. Slope of the graph would be 
( ${\left[\text{R}\right]}_{\text{o}}$ = initial concentration of reactant ; ${\left[\text{R}\right]}_{\text{t}}$ = concentration of reactant left unreacted after time ‘t’)