will not know enough to know which stocks to buy

they won't know enough to know when to buy them but they don't have to

because

they can buy all of america through an index fund and then they just have to be sure that they don't jump in at exactly the wrong time and they won't know what's exactly the wrong time so therefore they should put their money and

over a period of time and they'll have some periods that are wonderful and some that weren't so good but overall they will do fine over time