Well, if you already have Bitcoin then ignore this first paragraph. First you need some Bitcoin. You can buy bitcoin from several places such as Bitpanda or Coinbase.

Now that you have your Bitcoin you need to send it to a cryptocurrency exchange like Bittrex or Poloniex and from there you can look for the currency "steem". Put in a buy order and you should have your Steem. From here go to your exchange wallet and withdraw your Steem. Under the address tab type in your Steem name (not the nickname, the actual account name at the end of your URL). Then, accept the transaction and in a few moments you now have Steem in your Steemit wallet. From here you can choose to either hold it or power up your account.

I hope this helps, if you get stuck or have any questions feel free to ask away, I will get back to you as soon as I can.