Depends on the currency you want to deposit on Poloniex.
After sending your coins to the Poloniex deposit address, go to your "History" page and the deposit should appear in your "Deposit History" along with the number of network confirmations needed to complete.
Poloniex don't allow you to sell Bitcoin for USD. So, after trading your altcoins for BTC, you would need to withdraw your Bitcoins to another exchange like Kraken or Coinbase, and sell your coins there.
https://poloniex.freshdesk.com/support/solutions/articles/1000166427-where-is-my-deposit-