You need more information in order to do that. For example the size of the sheet of paper. Let's say you have it.
What you need to learn about is called "homography". This is basically the following situation:
You have the same planar surface (your sheet of paper) and you take a picture of it from two different cameras (Say one is the actual image that you have and the other is the one that you want to obtain - the one with the camera exactly above the sheet of paper).
There exists a transformation from 2D space of one image to 2D space of the other image (homography) and your goal is to find it. Once you find it you just apply it to your image to get the top view.
In order to find the homography matrix you need (at least) 4 points, whose coordinate you know in BOTH images.
An obvious choice for those points are the vertices of the sheet of paper of course. In the image that you have you can locate those by hand. In the target image, you can pick those such that the sheet is centered it (0,0), knowing the dimensions of it.
There is plenty of information about the homography matrix from 4 points online. This is just one of the first I came upon, so there must be better sources out there :)
Note that most often these computation are done in the 2D projective space, as this is a projective transformation.