Can you provide us with some more context?
do you mean claim your rewards? it's just a click on your wallet page on the "claim reward" button.
Or do you mean powering down? it's a 13 weeks process of converting all your Steem power into Steem with 1/13 of your SP being converted every week.
Is it changing steem to bitcoin? then you'll need exchanges, the most popular being Poloniex and Bittrex.
Or do your mean getting cash in your hand? that depends on where you are and what kind of financial tools you have with you. (banks, debit card .etc)
powering down and changing into bitcoin
Wrong move. Keep Steem for now
while i do agree with @melip,
for the sake of your question you can perhaps read this post by @cryptos here
https://steemit.com/guide/@cryptos/understanding-steem-power-and-how-to-power-down-your-steem
once you've powered down. you can sell your STEEM to an exchange. There are a few but personally i recommed Bittrex. There's a "noob-friendly" post you can read here
https://steemit.com/steemit/@nioctib/how-to-withdraw-steem-from-bittrex-to-steemit-wallet-noob-friendly
but I do hope you keep them though, as a starting Steemian your most wonderful friend is your Steem Power