For now, just use steemit, don't worry about buying the coin. You'll quickly learn more about how steem works (it's a popular posting topic at this stage of the game) and then you'll be able to make a more informed decision about whether to buy any. The economics of Steem is a complicated beast and is not subject to easy analysis, so give it some time before making a real decision on its investment potential. But enjoy the site in the meantime, and definitely take advantage of the low-hanging fruit of profits you can make just for posting original material (no investment required, other than your time).