weenis is a bot, so he won't respond ...

Easiest way is from your Wallet page, you click the little arrow on the STEEM line, and deposit - that allows you to send bitcoin and receive Steem or Steem Power in exchange.

Another method, is also done from your Wallet page : on Steem and Steem Dollars, the little arrow gives you the option to Buy or Sell.

Finally, there are a few exchanges who deals with Steem, , such as https://poloniex.com/ , which started trading Steem very recently , and https://blocktrades.us/ .