Hello Sandra - good to see you here!
Or, highlight the bit you want in a quote box, and look at the buttons just above the main box you're typing in. The one which looks like "" will put Gaius's codes at each end of the text, and when you post, hey presto, it will (should) be in a box.
If you want to make sure, click "Preview" instead of "Post" and you'll see if you've made it behave, and can then just post, or edit...
Emma