You just beat the Celadon City gym which is the fourth gym in the Kanto region. What next, what place is the fifth gym? You best have a bicycle for getting around now! In order to get to Fuchsia City you must have a bicycle for the simple fact that you have to go through Cycling Road. Cycling Road is a giant downhill slope that will take you from Celadon City to Fuchsia City. Many bikers will stop you along the way and challenge you to a battle. They are sort of strong but if your Pokemon are at good levels and were able to beat the fourth gym leader you should be able to handle it. By the time you arrive to Fuchsia City you should be able to head straight to the gym. However you might want to check out the Safari Zone.
The Safari Zone is a place where you can capture certain Pokemon. There is a fee to enter and you have to use Safari Balls to capture the Pokemon. You can’t just attack them, you actually can’t attack them sadly. You must throw bait. It’s not as bad as it sounds, it is actually not bad at all seeing that these Pokemon are pretty rare. But you should go and check out the fifth gym now. The fifth gym is a poison type gym. This really does give you a good hint that you will need Antidotes and Full Heals to cure your Pokemon of the poison status condition, because it does get in the way. This gym can be a pain because there is an invisible wall that you have to work your way through, meaning if you get caught in this maze you can end up fighting even more trainers.
Once you arrive to the gym leader, Koga be ready to face him, he is a ninja! Koga has four Pokemon, I believe it is the most so far, I think the last gym leader had three. The first Pokemon he will use is a Koffing, just play your cards right and you can defeat it, then he has a Muk, just take it down the same way as you did the Koffing and you can get it. He will then use another Koffing which is the same level as the first Koffing so I guess just battle it the same way. And the best for last, his strongest Pokemon is a Weezing. It is at level fort three. Everything should be going fine as long as you have the full heals or antidotes. At most you would probably have to use a healing item like a Super Potion. After beating Koga you will get the fifth badge, the Soul Badge. You only have three more to go, the next gym is in Saffron City!