To highlight text, just take your mouse and physically highlight a word or sentence, then you'll see a small box pop up with some symbols. The first symbol looks like a small pen, click that and it will highlight text.

And yes you're right, you have to be accepted first to a publication, but I've found that 9/10 they'll accept you unless it's a top-notch tech publication that only takes in established writers.

And they'll also have to approve your articles as well, most of the time they will accept them unless there's glaring spelling or grammar errors or if it's poorly written (you can always resubmit it). Sometimes they'll accept/publish it right away, sometimes it'll take 1-2 weeks depending on their queue.

Hope this helps!