Allowing a player to draw a shape which becomes a mesh...

Hi. I'm trying to figure out a relatively simple way of allowing a player to draw a custom shape with the mouse (like how you would draw in MS Paint and whatnot), then automatically filling in that shape with solid color and converting it into a mesh with collision. This shape can be a 2D plane if need be, but I would prefer it to be extruded to become a 3D shape. For example, if the player drew a circle (2D), it would extrude to become a cylindrical object.

I'm very much a novice when it comes to programming, and I have absolutely no idea of how to go about doing this (so feel free to explain like I'm five). I'm better versed in Javascript than anything else, so Javascript answers/resources would be most helpful. Even if you can only point me in the right direction, any help is greatly appreciated!

Thanks!