How do I make a fighting game?
You shouldn't. The OHRRPGCE is not designed for making fighting games, it is designed for making RPGs. Although it is possible to make a fighting game in the OHRRPGCE using plotscripting, this would be pretty hard -- maybe as hard as making a fighting game from scratch in a programming language, so it would be silly to attempt it.
If you want to make a fighting game, you might want to consider a game maker designed specifically for fighting games, such as M.U.G.E.N