1. How can I keep my Bitcoin safe?
Electrum cold storage optionHaving a cold storage. But If you do not know much about cold storage and also need an easier alternative, use a hardware wallet.
Open source hardware wallet.For small amount, you can use
Electrum.
2. Is there an alternative to centralized exchange?
There are decentralized exchanges. You can try
https://bisq.network/, but you strickly need a computer for it, but going for mobile option , but not as decentralized as Bisq, you can check this site:
https://kycnot.me/Note that the exchanges listed are no KYC exchanges, you can read the review on the site to know how perfectly decentralized or centralized they are.
2. There are decentralised exchanges and peer to peer ones. Both generally don't hold your funds in custody, and if they do it's only for a few hours to provide escrow - some send angry emails if you leave your funds on there for too long too. Localbitcoins always felt a lot safer than most exchanges (I'd probably rate it above binance and coinbase too for safety).
But Localbitcoins is not decentralized, it also requires KYC.