Exchanges ,Wallets and security

[barry89]

Im a novice here and have some questions hoping for answers.
With the uncertainty surrounding bitfinex, was wondering how reliable is cex.io.? Which exchanges uses tether? I thought bitfinex was trading in usd but its usdt and tether's been hacked.
Heard from an experienced trader -Instead of having your coins in exchange wallets, have your own wallet.
What exactly does this mean? How does one convert to fiat if you have your own wallet?
Is having your own wallet safe unless its in a hardware with you eg hardware crypto wallets?

As someone new and willing to invest in cryptocurrencies, what are the basic security measures to take (please list top 5).

Thanks
Bharadhwaj