First of all I would like to welcome you to Steemit!
I assure you there are a lot of users on here who are not any kind of celebrity or company. They're just regular people like you and me and that's just positive!

There is this third-party chatroom where you can talk to users alike (keep in mind never to share any of your private keys; especially the master key, to anyone nor any site suspicious).
You can find the chatroom by the domain "Steem.chat" (that's the whole link).

I also recommend you to read through different tutorials and guides people have made about how Steemit works.
For example, @sykochica has made a lot of simple yet relevant guides and gathered them together into a google docs link in her page's header.

As of what to post on Steemit - well, you can post anything at all, really! Your thoughts on some topic, your life, your art of any sort... Just remember not to share anything you'll later regret sharing (personal information), as what's published on the blockchain, never gets removed.

Good luck and stay safe!