It is best to buy ETF's such as the SPY, QQQ, XBI, and China ASHR. Even EURO STOXX 50
ETF's are the safest because you never can never tell what will happen in the future with individual stocks such as META for example.
ETF's as mentioned you can be sure will eventually go up and get your money back, individual stocks, not so much.
If you want more bling go for leveraged spy or qqq. They can be more painful in the short run but hold their core value and recover the same as regular ETF's.
You can never call the exact bottom but look for a bottom to hit before you buy a trauch, such as SPY 350.
If you buy individual stock i suggest you always set a stop loss of 2-5%. And raise this as your profit increase.