Find the post that has a quote that you like, hit Reply with Quote on the right, then highlight the part that says something like [Q UOTE]blahblahblahblah[/Q UOTE] and right click/copy. (space in the word quote so it doesn't actually quote me, lol)

Then looks like know how to edit your signature already, under the Settings button at the top right of the forum page. Go into Edit Signature and paste what you copied from the original post.