Which private key? There's four. Hopefully not the master key (which nobody should do but a lot of people do).

The keys are in the wallet in the Keys and Permissions tab. Use the posting key to log in and do stuff on any given interface (including steemit) and use the active key for transfers. Don't ever use the master key again.

Except to change your other keys and then do that super carefully XD

While you're about it, grab the steem keychain extension for your browser and use that to log in, it's a bit safer than popping it into the site. Even if it is steemit.