Hey yulan,
welcome to Steemit. I wish I would know about such things when I was in your age! Wish you the best for you future.
By the way here is a nice link which describes it pretty well how the ecosystem of Steemit works:
https://steemit.com/steemit/@donkeypong/still-confused-by-steem-steem-dollars-and-steem-power-the-power-plant-analogy
If you have further questions. Here I am :)!