If you wish to purchase L$ to spend in-world, then click Buy L$ at the top of the screen, right after your balance display, or go to top menu bar Avatar → Buy L$. The window has a few fields:
Click Buy Now to make the purchase, or Cancel. If you do purchase, the funds will be taken from your payment source, the one you have on file with LL. This implies that you need to have payment information on file. You can add this information by going here.