Where does STEEM come from?

The issuance of new STEEM is created by the protocol (i.e. planned inflation). The new STEEM is divided between miners/witnesses and participants.

Steem is a blockchain project (similar to Bitcoin). Miners/Witnesses create new blocks with transactions (posts, upvotes, transfers of STEEM currency). As a reward for that work, miners receive STEEM that is newly created by the Steem protocol (network rules). There is also a reward of newly created STEEM set aside for participants in the network to pay them for curation (upvotes, reblogs) and authoring (posts, comments).

What is the value of STEEM?

The price is whatever people are willing to pay for it on the open market (whether with fiat or bitcoin). The value is what you can do with it -- whether on the Steem network (e.g. Steemit) or in the wider world (e.g. maybe buy a cup of coffee, pay for developer services)