There are a few ways you could draw a filled circle in MonoGame. First, you just simply import a png/texture of a white-filled circle and when drawing you could simply just set the color in the sprite batch draw call, this would probably be the easiest method of doing this. The other way is manually creating a texture at runtime and filling the individual pixels with color. If you wanted to go this route, look at this post as it seems to do exactly what you want: c# - XNA Draw a filled circle - Stack Overflow