The best and optimal method would be to avoid exchange since they will take their fees plus the fees taken by the miners that you gonna have to pay to send the coins to them on the blockchain, and also you gonna have to give them your personal information (KYC Know Your Costumer)
Exchanges are still the most simple, I suggest Coinbase who only require 2 euro of minimum deposit.
If you want a cheaper, savy way, more optimal and more bitcoin way of trading your bitcoin you could try local bitcoin which allow you to met other person willing to trade bitcoin in real life, or Bisq which is a decentralized exchange software.
