You can buy it on serveral different markets (where you can buy it direcly in USD or convert another cryptocurrency to Steem) and you can get it from upvotes to both your posts and comments. Note that not all upvotes pay the same on Steemit, to better understand that I suggest you to take a look into the Steemit FAQ here: https://steemit.com/faq.html#Can_I_earn_digital_tokens_on_Steemit_How
Good luck!
RE: How can I earn steem on steemit?