What is a mask? A mask is an image in black and white (usually, anyway, this is how this site works) where each pixel actually represents a depth marker. The whiter the points, the closest the point are to you. So, black represents the background, and the different shades of white represent points floating above the background.
Its pretty difficult to make a mask for your self from scratch (uploading a picture of your self will not make a good mask, this I can assure you). To do this, you would have to use a tool like 3D Studio that allows you to make 3D objects, and then to generate a mask based on your 3D object. You can however create simple masks with basic tools like Microsoft paint, especially geometric shapes. You can also make text masks using the on site text based mask generator. It’s simple, just enter the text, and that’s it.