Before you get the ruby you must make sure that you have beaten the pokemon leauge AND you have to catch 60 pokemon and talk to Prof Oak, then go to One Island by going to Vermillion City. Once you get there go straight to the Mt. Ember entrance, make a lfet turn and then you will see two Team Rocket grunts blocking some sort of cave, battle the Team Rocket gruts and beat them, then go inside the cave. Just keep going until you reach a spot where you see a red gem, that red gem is the ruby that you have been serching for. Give the ruby to the guy at the big machine in the Pokemon Center at One Island, give him the ruby and he will give you a Rainbow Pass that will take you to the Sevii Islands.