First of all you should know that you can actually buy some steem from two main trading sites: Bittrex and also OpenLedger. Personally I prefer to buy it from Bittrex because its more easier to understand the trading process, OpenLedger is a bit complicated for a newbie in the trading world !
To begin with, you need to buy some bitcoins on coinbase.com, for example If you want to buy 10 $ of Steem knowing that a unite of steem is valued at 0.241 $ you will buy 10 $ of BitCoin ( just put the amount in dollars of BTC you want and site will tell you how much BTC you're buying ;) ) , after that you will need to create an account on Bittrex with the mail, the password, etc but it seems like there's a problem with validating the account but don't worry it still let's you trade. Once you created the account, go to the "Wallets" section of the Bittrex ( look at the top right) on "ACCOUNT BALANCES" you will have the names of the cryptocurrencies traded on the site, click on the "+" sign next to Bitcoin then on "generate new adress" copy the adress ( its the adress where you're going to send your Btc, it will be deposited on your Bittrex wallet ) then go to coinbase.com , section "Send/receive", put the adress on "recipient", and the amount you want to send to this account it will be shown on your Bitcoin wallet click on "send the funds", and then you will have to confirm.
Next you will have to wait a half an hour approximately to be capable to use these Bitcoins, then go to "B Markets" search for Steem, next to the chart you will the Steem price compared to Bitcoin on the first line, the "Bid" section shows the lowest buying price and the"Ask" price shows the lowest sale price, as you want to buy some of it you will look at the "Ask" price, you can get immediatly your Steem if you buy it at that price and NOT LOWER because you're going to wait a while ( I personally think Steem will hardly fall in price so I guess that the actual price of 0.24 $ is the best price we will ever have ), next go on "Trading" part, then enter the lowest "Ask" price you will have ( second line) on the left board, on the first line put the amount of steem you want to buy and on "Total" side it will show how much its going to cost in BTC ( for example you have bought 500 Steam at 0.00054 BTC per Steem, it will cost you 0.27 BTC ), you will have the "Good Till canceled" kind of buy activated if you don't know what this is, click on link "What is it?" they explain it well.
Yay ! You finally made it, you bought some Steem congrats ;) !
Hope this helps you guys !