First and foremost, the purpose of quoting is to make it easy for other readers to understand what you're talking about. Quoting the whole of a long posting is unnecessary if you're only replying to one particular point. Worse, it's frustrating for the reader to scroll through a lot of stuff they've already read once (or even twice, if the quoted posting itself contains quotations), to try and find exactly what the replier is replying to.
So two basic things to remember:
a) If you're quoting using the "quote" button at the top right hand corner of the posting you're replying to (Method 1 below), please edit out anything that's irrelevant to the point you're making - though without distorting the person's original meaning;
b) If you're replying to something in the immediately preceding posting, it may well be unnecessary to quote unless the posting was a lengthy one - so try to avoid doing this. Even if you judge that it really is necessary, you should still be selective in what you quote.
Right, on to the "How To" bit...
There are two methods:
1) The simplest way of quoting is to hit the "Quote" button at the top right-hand side of the posting you want to quote. This takes you to a "Post a reply" window with the earlier post quoted at the top:
[quote="(name of poster)"]...Text of post...[/quote]
You can then type your reply in that box, having edited the earlier post as necessary so that you're quoting only the bits you need to.
2) The other way of quoting is to hit the "Post a reply" button, then use the "Quote" button (fourth from the left among the buttons immediately below the subject field) to insert a [quote] tag. After you've inserted the text of your quotation, hit the button again (which will have changed to a "*Quote" button after you hit it the first time) and it'll insert a [/quote] tag.
(Or, more simply, you can highlight the text you've pasted in and hit the "Quote" button once.)
By the way, if you want to attribute the quotation, you can add the username afterwards in this way: [quote="username"]. So if I wanted to quote Kay, I'd put [quote="Kay"], and my quotation would be headed "Kay wrote:". Be sure to give the username exactly as it appears on the post (including capital letters where appropriate) because otherwise the system may not recognise it as a valid username.