Look at the bottom right corner of this or any other post. Press the 'quote' button and you're on your way. You'll note bounding [-quote-] and [-/quote-] tags (minus the hyphens) and you can add more of those to split a quote up and add your comments between different parts of the quote.
If you want to quote more than one person, click on the 'multi' button in each of the posts you want... and then go to the bottom and press 'reply'