About how to independently manage accounts?

I am completely new here, so go easy on my question, as I am just doing some out loud thinking.

I have some relatives and friends with 401k's, individual accounts, and just some money sitting around. They are constantly asking for advice, where to put their money, what funds, when to balance, and everything else they have no clue about. Several of them have said "why don't I just give you the money and let you manage it?" I told them because it wouldn't be legal.

My question is, what would be the simplest route a person can take to open an account and trade on behalf of a person. Where would be the best place to begin looking into this, and what licensing/certification would be the simplest way to be able to do this?