You can also just click the Quote icon

which will place the quote / end quote brackets within your message,
and then, after copying the text you wish to copy as wt1 says,
right click / Paste in between the [quote ] (Paste To Here) [/quote ] 's.
Always nice to use the Preview button below the Message window to see how it's all going to look come Christmas morning also.
Using any of the editing buttons on top of the Message window will not be the 'No, don't press That button, end of the world scenario' at all really. All can be changed / edited after the fact, even after you press Post New Reply.
Just click the editing icon on top of your reply

to access your message or, on the first post of a topic, your title too.