I have a post on the trading experience here. That is dealing with Steem Dollars but the experience is similar with STEEM. Not sure what exchange you're using though, that would be a helpful detail to add.

Transferring to your steemit account will be by your username. Memo can be anything you want it to be coming in. Going out to an exchange it will have to be a very specific value that they provide to disambiguate transfers between users.