1. A 100w light bulb produces 8w of light
What happens to the other 92w?

What other forms of energy do you know about? Which of these could be generated by the light bulb also?

Erm... radiation as heat?

Yes that's the main one. Pretty much all of it is heat.

If you think about it, in order to emit mostly visible light, it has to be as hot as the sun. Which it isn't, and so it emits in the infra-red, which is heat.