How do i fix the bottom screen of my nintendo ds?

When i turn my Nintendo DS(Its the original one, the really thick one http://www.n-sider.com/media/database-hardware-nintendods01.jpg ) on, i can see the warning/safety thing, but after touching that, i have a hard time seeing what the bottom screen says. When i play a DS game, it gets worse. Does anyone know what's wrong and how i can fix it?Thanks.