You can buy or sell ada for fiat or other cryptocurrencies using cryptocurrency exchanges. Visit coinmarketcap to see the list of exchanges that support ada.
As an ada holder, it is important to keep your funds secure, and that means you need to keep your private keys private. It is highly recommended to avoid keeping your cryptocurrency in an exchange longer than necessary, and instead to use a cryptocurrency wallet. We recommend either a full-node wallet like Daedalus, or the browser-based Yoroi.