The first thing to remember when quoting from another message is: do it sparingly. If Mrs T has written a long rambling message, but you're only replying to a single sentence, you should only quote that sentence.
The easiest way to include a quotation from her message is to post your reply in the same thread. Instead of clicking the "post reply" button to start your reply, scroll to Mrs T's message and click on the "quote" button. This will bring up a new message box, including the whole of Mrs T's message. If Mrs T's message is quite short, you can just type your reply below, and press the "submit" or "preview" button as usual. Usually, though, it's better to edit Mrs T's message. To do this simply highlight the bits of Mrs T's message that you do not want. Then click the delete button on your keyboard. Warning: be careful not to delete the bits in square brackets at the beginning or end of Mrs T's message. If you do this, the quotation won't work.
If you are quoting Mrs T's message in another thread, or (let's pretend) quoting Billy Bragg, it takes a wee bit longer but is still pretty straightforward. Start your new message as normal. The key is to use the formatting tools at the top of the box.
The first step is to make your text bold. Click the "bold" button. Now type something like "Billy Bragg once said". Click the "bold" button again - this stops the bold formatting. Click the "quote" button and type "When the world falls apart/some things stay in place". Click the "quote" button again - this shows that you have finished the quotation.
Now type the rest of your message. When you hit the "preview" or "submit" button, you will see the quotation in a box, with "Billy Bragg once said" at the top, in bold.
If the message looks wrong, it's probably because you have accidentally deleted some of the formatting, or forgotten to press the "close" button.
Use this thread to share your favourite quotations. It's a great way of practising -- and no-one is going to complain if you don't get it right every time!
BTW: please avoid putting quotes inside quotes. It gets very confusing.