How can I convert 3D space coordinates to 2D space coordinates?

I'm using a Lua API. I'm trying to transform 3D space coordinates to 2D space coordinates. I've asked google but I can't find anything apart from snippets using OpenGL function. I don't need source code or anything just a simple way on how to do it? I can acquire the Camera's perspective, the original position to be transformed, window size and aspect ratio if that's any help? Thanks in advance for any suggestions :)