Welcome to Steemit! Go into your wallet and then the permissions area. In there you'll see your public keys.
Use the Private Posting key for day to day steemit use like replying to posts and authoring posts.
Use the Private Active key to make transfers of your crypto, etc.
The "owner" or master key is the primary key for the account, and it should be kept offline and not used on a day to day basis as much as possible. People should print that key out in a font that makes sure you can read it easily. Then store it in a safe place. If your account is ever hacked, that is the key used to recover it. The key is also used to change the other ones. Any time you think your account may be compromised, change your keys using the master/owner one. Always save your previous master/owner keys too. Having an old one helps in the recovery process as I understand it. If you make a habit of printing them out onto paper, that should be easy to do.
The memo key is used for purchases of crypto. For example, when I transfer from bittrex into my account, I use the memo key along with my account name to let bittrex know where to send the coins.
I hope that helps a little. It is very confusing at first. The main mistake people make, and I made it too, was to keep using your master primary key for everything. Do not do that! Stop using that primary key, print it out, put it in a safe place, and then don't be typing or copying and pasting that key on a regular basis to use steemit. The more online that key is, the more likely the key will be stolen.