We study the metric projection onto the closed convex cone in a real Hilbert space generated by a sequence. We provide a sufficient condition under which this closed convex cone can be more explicitly described. Then by adapting classical results on general convex cones, we give a useful description of the metric projection of a vector onto such convex cone. As applications, we obtain the best approximations of many concrete functions in $L^2([-1,1])$ by polynomials with non-negative coefficients. This talk is based on a joint work with Zipeng Wang.