w=10;
eqn1=y''[t]+w^2*Sin[y[t]]==0;
eqn2=y[0]==1;
eqn3=y'[0]==0;

sol=NDSolve[{eqn1,eqn2,eqn3},y,{t,0,1}]

Plot[Evaluate[{y[t],y'[t]/10}/.sol],{t,0,1},PlotStyle->{Blue,Red}, PlotRange->{{0,1},{-3,3}}]