“Bitcoin” or cryptocurrency exchanges are good places to buy or sell Bitcoins with different currencies.
Coinbase is a well-known exchange and cryptocurrency wallet, which is easy to use.
In terms of security, it is just no different to a bank where they store your password, personal details and your private key to your Bitcoin wallet.
Etoro and Bitfinex are notable cryptocurrency exchanges in the UK & US that most people buy Bitcoins from.
Platforms like Coinbase, Etoro and Bitfinex are prime targets for hacks so be careful of storing your cryptocurrency on exchanges.
Bitfinex got hacked in 2016 and had US $72 million dollars worth in Bitcoins stolen from them.
Good guidance