A memo is the only way to send a message to someone on Steemit without leaving a comment, and it costs a very small amount of STEEM or STEEM Dollars to send it. Here's an example of one I received recently:
In the tab "My wallet" you can click on the dropdown arrow next to your STEEM or STEEM Dollars and hit transfer. When you click on transfer you can select another user to send the memo to, as well as the amount of STEEM or STEEM Dollars you want to send and the text of the memo itself.
As long as you aren't sending memos demanding money or containing malicious links you should be okay :)
I hope this helps!
RE: Poor Man's Steemit Guide: My #1 Tip for Getting Started Fast on Steemit