How to create (N + 1) x (N + 1) vertices in xy-plane with OpenGL

I'm trying to draw a grid made of triangles with the inmediate mode of OpenGL but I'm struggling with it. Particularly the number of vertices has to be (N + 1) x (N + 1) and "linked" between 2 x N x N Index-vectors. What I've got at this point is the following code:

void generate_grid(
    std::uint32_t N,
    std::vector<glm::vec3>* vertices, //vector which contains all the vertices
    std::vector<glm::uvec3>* indices) // vector which contains all the indices
{
    /*cg_assert(N >= 1);
    cg_assert(vertices);
    cg_assert(indices);

    vertices->clear();
    indices->clear();*/

    std::uint32_t size = 2*N*N;

    glEnableClientState(GL_VERTEX_ARRAY);

    glVertexPointer(2, GL_FLOAT, 0, vertices->data());
    glDrawElements(GL_TRIANGLES, size, GL_UNSIGNED_INT, indices->data());

    glDisableClientState(GL_VERTEX_ARRAY);

}

All your advices about it will be well-recibed (I'm in process of learn OpenGL, be kind ;)). Thanks in advance.