Ok, I know that I am presupposing two things when I challenge your belief.
I presuppose, that you believe in gravity, and that you believe that the Earth is placed in a vacuum filled Space.
Now, to somehow prove that the Earth is round, I will take a drop of water as an example.
Have you ever studied the shape of a drop of water in freefall? I'll be so kind as to provide you with a couple of pictures:
http://misheli.image.pbase.com/v3/29/547829/1/47304883.IMG_6591.jpghttp://k41.pbase.com/u18/ronnie_14187/upload/36165836.WaterDropsLatest50.jpgAs you can see, the tiny droplet of water in each picture is distinctly round in shape. This is because matter attracts matter. Now, since there is a certain amount of water molecules in said amount of water in freefall, these molecules will be attracted by each other, leading to the given amount of molecules joining with each other, shaping the drop of water.
There is a reason why the drop is shaped as it is.
If you draw a circle on a piece of paper, and measure the radius from any part of the periphery of the circle, you will find, that the distance from the periphery to the center of the circle always is the same. This means that anywhere on the periphery, the distance to the center is as short as anywhere else on the periphery.
Lets project this picture to the drop of water. Each molecule will travel towards the center of the drop with equal force. If each molecule pushes with equal force, the droplet will take the shape of a sphere.
Now, let's look at Earth. The Earth is an amount of matter existing in an immense vacuum filled space. The Earth is as close to free fall as anything. Even more than the drop of water I spoke of, since there is no air to break the fall of it, and misshape it. Let's assume, that gravity not only works for small amounts of matter, like a drop of water, but also for large amounts, like Earth for instance. If the same rules that apply for the drop of water, apply to the Earth, then logically, the matter of the Earth would travel with equal force towards the center of the Earth, making the Earth round!