Basically you just post things, or comment on things and when people "upvote" you, you get a bit of STEEM, which you can later transfer to money. Depending on how much STEEM the person upvoting you has the more STEEM you will get from their upvote. You upvote by pressing that green arrow next to your name and all the other doodads below your post. You can also get STEEM if someone upvotes your comments. Don't hold your breath on getting any actual money though as there is already a well established group of people who make the majority of the STEEM here judging by what I have seen and heard.