Question: what to do after beating koga in pokemon yellow?
Answer: Once you beat Koga, HM 03 (Surf) is unlocked for use outside of battle. Teach Surf to a pokemon and you will be able to move in water. So if you have not yet defeated Sabrina in Saffron City, that will be your next thing to do. If you have already beaten Sabrina, then your next stop is Cinnabar Island.