To Quote, click on the "Quote" button on the bottom-right of someone's post.
Simply copy this by pressing "Ctrl + C" on your keyboard, and paste by pressing "Ctrl + V".
At the very top-left of your page, there is something called "User CP". Click on the drop-down arrow next to it, and go to "Edit Signature". Here, simply paste the Quote (Ctrl + V), and click on "Save Signature" below. =]