You should read the tutorials first.
http://irrlicht.sourceforge.net/tutorials/
For example tutorial 8 shows how to add a light.
http://irrlicht.sourceforge.net/docu/example008.html
For more information, you can look into the documentation of light scene nodes:
http://irrlicht.sourceforge.net/docu/cl ... _node.html
They show a lot and reading them will give you an idea of how irrlicht is structured.
Then you will find things easier in the documentation (
http://irrlicht.sourceforge.net/docu/)
P.S. You write, that irrlicht is a game engine, but it is 'only' a 3d engine. This means it does not have things like advanced physics (but it has a simple collision detection).
For many purposes, you'll have to use some additional librarys (or code the stuff yourself, which I do not recommend, because there are many good librarys out there).