If you want to reply to something specific from a post above, just click the 'Quote' button

You'll see something like this
Simply write whatever you like underneath

When you're done click 'Submit' and your post will appear

Editing what a previous poster has written
You can also pull out parts of a previous quote, but make sure you always have this tag at the start

And [/quote] at the end...

Multiple quotes in the same post
You can include as many quotes in a single post as you like. Here's an example

Good form
When quoting, try to quote only the part of the message you need. Otherwise you end up with quotes within quotes within yet more quotes and it's hard to read
If you see lots of quote boxes cluttering up a thread, do us all a favour and hit 'reply' instead