Why does my picture has a shadow in the middlle?

Hi. I'm an amateur photographer and recently acquired a nikon d90 camera, i'm a baby at photography so i don't really know the fine features of my camera yet. My concern is that on some of the pics i've taken there's usually a circle shape shadow in the bottom middle portion of the picture, is my camera busted, or am i doing anything wrong?

The problem is probably cause by one of two things.

1) your lens is casting a shadow because you are using the on-board flash unit
2) your lens hood is casting a shadow. This can happen even if you are using an external flash attached to your hot shoe.

Nothing is wrong with your camera!

If your taking a picture with flash, sometimes considering the angle your taking the picture from, there could be a black semi circle at the very bottom of the photo, and if thats what you have then it shouldnt be something you should be too worried about

Best to show it to have an idea. Could be a flare, or could be your finger at the edge of the lens. Or some failure