There are a few ways…
Hit the reply button beside someone’s post. That will show that you’re replying to them top right hand corner of your post.
You can @ them (tag them) in your post. Example, if I @Name then “Name” can see this post.
Or to quote them, highlight the text of theirs that you want to quote and hit reply beside that post. You’ll get a pop up in the blank text box on the same page and then you just plug in your reply and hit ‘reply’.