If you have coins sitting on an exchange, you can "withdraw" them by putting them into cold storage. You should never keep BTC with an exchange (essentially a 3rd party), for long periods of time. Taken them off the exchange, and put them into
"cold storage" so that you are the only person who can access them (form any country, anytime, any location, provided you have internet access and your "private key" (the secret code that only you should know, that enables the coins to be accessed).
If you wish to sell BTC, (all, or any portion thereof), you can simply arrange for a buyer to give you cash, goods, services, etc, in return for you transferring an agreed about of BTC to them.
The term "cashing out" i typically used to refer to converting BTC back into fiat currency.
PS - if you are new to this, please do not get scammed. Do not fall prey to internet scams. Buy from a reliable exchange, or from a person or business that you know is creadible.