You'll use your mouse. Point just past the end of the text you want to copy, press and hold the left mouse button and drag the mouse pointer over the text you want to copy. When it is highlighted, let up on the mouse button. Place the pointer over the highlighted area and click the right button once. You should get a menu. Click on Copy.
Then go to your > user cp and Edit Profile to where you enter for your signature. Point the mouse where you want to copy the text and first click the left mouse button to put the cursor where you want to paste and then hit the right mouse button again. You should get the menu again and this time Paste should be an option. Click on Paste and the code should be there. At the bottom of the page, click the Submit Modifications button and that should do it.
You can always go to the signature area and just type the same thing but it is always good to know how to copy and paste. Comes in handy in a lot of things.