How to replace motherboard to a computer?

How to replace motherboard to a computer?
I bought a desktop computer with an AMD Athlon 64 3500+ processor, 200 gig hard drive etc. It recently started displaying very wavy images on my screen. I turned it off for a while but now when I turned it on my computer, the fans just start up and keep going and no image is displayed on my screen.
I replaced the graphics card and still have the same problem but I didn't hear beep sound or anything. I was told by my friend that it was the motherboard problem. What should I do to fix this? Does anyone know this kind of problem?
