you should know first what altcoin you have on exchange where, when you already know it you can sell altcoin that you have to the exchange, because not all exchange there is altcoin list that we have.
yes that's right, before you have to see where you can sell. because when the exchange is not available, you have to wait until then. and immediately sell before its value becomes worthless, because not all the coins that appear will have potential so you can sell it higher.