Poloniex is surely a good idea. Once you've set up your account you'll have to check the subpage "deposit and withdrawal". Go to the line "Steem" and click on "deposit". Then copy the "memo" (key).
Then go to your steemit wall, click on the button close to "Steem" and chose "transfer". Sent the respective "amount" of Steem to "poloniex" and the "memo" you just copied.
Once your Steem has arrived at your polo account you can exchange it e.g. into Bitcoin. That happens on the "exchange" subpage. I use "localbitcoin.com" to exchange BTC into EUR then, so send my BTC to another platform. But that's only because in Europe we have few other possibilities to exchange crypto into fiat. There may be other, easier ways for you like the ones mentioned here already.
Best of luck :)