What career or courses do I need to take to be an ambassador??

Ambassadors are appointed either due to their public recognition or due their political bias. There is no subject or study to qualify for this as Ambassador is not a job. So you can go for any study you want and either get succesfully politically involved or build up a strong public image. Maybe political studies would help as you may have a better chance to work closely with political parties to build up their image campaigns.