Don’t buy penny stocks;
don’t buy stocks all at once (portfolio and cash management);
don’t look at the previous high and use it as reference price when buying (on sale mentality);
don’t own too many stocks especially for newbies, because it’s likely a newbie doesn’t know or learned a lot about those companies. In stocks we want to know something other people may not know, e.g., quite a few people don’t know or realize that Activision Blizzard $ATVI is posed to receive the $3 billion breakup fees from Microsoft if the antitrust review does go through. Huntsman corp $HUN got a similar jackpot from PE firm when the firm wanted to back out from a deal;
consider S&P index funds as the default investment option for 401k/403b/IRA etc. There are quite a few such funds in the market, note I am not talking about ETFs. Personally I have those funds from fidelity and vanguard, and I know others offer similar funds too;
be greedy when others are fearful and be fearful when others are greedy (VIX is a decent indicator for the turmoil/fear in the market).