Warworld is the first part of the game, on his way back from his home world Superman is abducted and must fight for his life.
It's an easy enough level, you're fighting in a dome and have objects you can pick up all around you, you'll have to defeat 2 mini-bosses and then you'll be up against Mongul.
Rather then type up an explanation on how to beat each boss I've linked in some videos showing you.
* Credit to dragonball1957 for the video