My Nikon D40 won't auto focus on a white background?

I'm trying to take photos on self-timer and auto focus in front of a white background but it will not focus at all. I'm not very good using manual so it's just blurry when I switch to manual. Can anyone help me? I need to take pictures of myself but can't get my camera to focus!

It can't focus on just white. Put something where you will be standing and then it will have something to focus on.

White? Well that's understandable. Cameras need things of contrast to focus properly and a white background isn't that. If it were of various colors you'd have better luck.

Tell you what. Stand a tripod, a tall floor lamp, some object like that in front of your background exactly where you're going to stand. Focus on the object, then move it out of the frame, stand in and let the camera fire. But be sure to use manual focus so the camera won't try to change focus before it fires.

Jim omitted an important step.
Auto focus (this is when you press the shutter release half way) on something at the same distance that you will be from the camera. Then, switch the lens to manual focus mode. The switch to do this is on the side of the lens and is discussed on page 8 in your manual. When you take your picture with the self timer, you will be in focus.

As others noted, the focus mechanism requires contrasting edges to autofocus. A solid color lacks contrasting edges.

Nothing does.

You're focusing on the background then shoot yourself in front of it? Of course you will come out blurred. Mount the camera on a tripod or put on a stack of books then set the self-timer on. Set up a chair with a broom or dustpan sitting on it. Autofocus on the broom or dustpan. Press the shutter release button then run into position. Do not push away the chair, just knock the broom or dustpan off. Sit, pose, hold your breath and smile. Repeat until satisfied with the results.

Of course it won't! It needs a subject to focus on; they usually work by assessing the difference in contrast but with a white background, there's no contrast.
If you want to take a self photo, either:
Get someone else to sit/stand while you set up the camera, then change places before the self timer takes the shot.
OR, get someone else to take a photo of you.