The easiest thing to withdraw is SBD or Steem. When you post and get rewards, currently on SBD and Steem Power are paid out.

If you have an account on Bittrex or Poloniex you can transfer the SBD or Steem there. Just go to your wallet and click transfer and you will send it to poloniex or Bittrex with your account number.

Here is a detailed post about it:
https://steemit.com/steemit/@ninjace/how-to-transfer-steem-dollars-from-steemit-account-to-poloniex

Steem is different than Steem Power.

You can convert your Steem Power to Steem, by going to your wallet and clicking Power down on the steem power. You will lose your voting power.