If you want to quote someone, there's a button at the top-right-hand corner of the post you want to quote that says simply "Quote". Just click on that and it will pull up the post page with the quote code set up for you.
You can also look at the posts under the one you're typing once you're at the post page and click on the red "Insert Quote" text, and it will add a quote underneath anything you may have already written.
Also, the code for making a quote from somewhere other than the forum is this.
[quote]Remove this text and fill with whatever you want to quote.[/quote]