R=1000; c=100*10^-6; u=10;

eqn=R*i'[t]+1/c*i[t]==0

sol=DSolve[{eqn,i[0]==u/R},i,t]

Plot[i[t]/.sol,{t,0,3},PlotRange->{{0,1},{0,0.01}},
AxesLabel->{t,i}]