How do I Fix my screen?

The screen on my iMac has a small problem. On half of it, I have lines of random colors running down my screen. The main colors are Green, Purple, and Yellow. They sit on top of all my window, and different applications. My computer isn't all that much slower now than before, but there is a slight noticeable difference. But I'd like to know how to fix it. If i need a replacement screen, or if i just need to tweak the Mac a bit. Any and all help will be greatly appreciated.

Thanks