"Do I always have to start the words 'king' and 'prince' with a capital letter?" If it's part of the title of someone's name, then they follow the same rules as for Mr, Mrs, Doctor, etc. eg. King George I, Prince William. Also, if it's the only descriptor of a person (ie. in place of that person's given name), then a capital letter is needed. If it's used as simply a noun, then no capital needed.

Thank you all so much.

Hello, Hamadia: Great question! It depends on what English-speaking country you live in and on the book that your teacher uses. Here in the United States, we no longer use the uppercase (use the capital letter) when referring to a person's position. So Americans would write: "While I was visiting London, I saw the queen waving from the balcony of her palace." They would also write: "While I was visiting London, I saw Queen Elizabeth ...." (They uppercase the word as a title.) I think (repeat: think) that people in England would show more respect and uppercase the word in all sentences: "When the Queen visits the United States, she ...." Today most American newspapers no longer uppercase "president." For example: "Thousands of people welcomed the president at the airport." But, of course, the title is uppercased: "Thousands of people loved President Washington." When I was young, newspapers DID uppercase "welcomed the President at the airport." But MOST American writers no longer do that.