Hey! You were close, but there is a small difference. "I will be there after one hour" means that after one hour has passed I should arrive. "I will be there in one hour" means that I should arrive in some time before one hour passes, but close to a full hour. Even though they are slightly different, they are both usually used the same way, so either one can be used. It is also more natural to say "an hour" instead of "one hour" unless someone asks specifically "How many hours will it take?". If you want to say that one hour is the period of time you will be there, you would say "I will be there for an hour". I hope this helps! Let me know if you have any questions or want me to clarify!