How do you go to koga in Pokemon Yellow?

Answer 1

Koga is the fifth Gym Leader, located in Fuchsia City. I'll assume that you've made it as far as the first four badges. First, you'll need to clear Team Rocket out of their base beneath Celadon Game Corner. Once you've defeated Giovanni and claimed the Silph Scope from him, head back to Lavender Town and climb to the top of the Pokémon Tower. Defeat the ghost of Marowak that haunts the tower and the Rockets that are at the top, and Mr. Fuji will give you a Poké Flute. Armed with this, head back to Celadon City, and go west to awaken the Snorlax. Battle it, and then go south down Cycling Road; Fuchsia will be directly east once you reach the bottom.

On a side note, you'll need a Bicycle in order to go onto Cycling Road. To get it, talk to the President of the Fan Club in Vermillion City to get a Bike Voucher, and trade it in at the Cerulean Bike Shop for a Bicycle.

If you mean how do you get to Koga in the Gym, due to the almost invisible walls that are there, go all the way to the east side of the gym, then straight north to the northeast corner, then straight west to the northwest corner. Then take three steps south, two steps east, two steps south, one step west, four steps south, and three steps east. You will be adjacent to Koga, but bear in mind that it is necessary to fight at least one of his subordinates to reach him.