Does it cost anything to post, comment, or vote?
No. It is free to post, comment, and vote on content on Steemit.com. You might even get paid for it!

Can I earn digital tokens on Steemit? How?
You can earn digital tokens on Steemit by:

Posting - By sharing your posts, you can earn upvotes from community members. Depending on the upvotes you receive, you will get a portion of the ongoing Steem reward pool.

Voting and curating - If you discover a post and upvote it before it becomes popular, you can earn a curation reward. The reward amount will depend on the amount of STEEM Power you have.

Purchasing - Users can purchase STEEM or Steem Dollar tokens directly through the Steemit wallet using bitcoin, Ether, or BitShares tokens. They are also available from other markets and exchanges including BlockTrades, Poloniex, Bittrex, Shapeshift.io, and Changelly. STEEM tokens that are powered up to STEEM Power earn a small amount of interest for holding.

Where do the tokens come from?
The Steem network continually creates digital tokens to reward content creators and curators. Some of the newly-created tokens are transferred to users who add value to Steemit by posting, commenting, and voting on other people's posts. The remainder is distributed to holders of STEEM Power and the witnesses that power the blockchain.

Where does the value come from?
At its root, Steem is simply a points system. However, because this points system is blockchain-based, the points can be traded on markets as tokens. People buy and sell these tokens, and many hold in anticipation of increased purchasing power for various Steem-related services.

By analogy, Steem is a game system where users compete for attention and rewards by bringing content and adding value to the platform. The rewards people earn are tokens that have market value and are readily tradable. It is similar to how someone playing a video game could obtain a limited item or currency by playing the game. If the currency or items are transferable between users, then they can sell or buy them on game item markets.

From here: https://steemit.com/faq.html#Does_it_cost_anything_to_post__comment__or_vote