Who has the power to add new states?

My name is Kevin ans I am indeed more than happy to help. I'm in 7th grade just took the Constitution Test. (Much easier than I thought) I'm going to guess that the Secretary o State might have something to do with the process of how a piece of land becomes a state in the United States. The Secretary of State deals with foreign affairs so if we want to buy land from a country they might be the ones to handle that. Only a guess.