Prince William will get the throne, since he is older and therefore has a higher place in the line of succession.
Currently it is Prince Harry until William has a child
Prince William is second in line to the throne, Prince Harry is fifth in line.
William is older, and so ahead of Harry in the line to the throne.
His son, Prince George. After George is William's daughter, Princess Charlotte, and after HER is William's uncle, Prince Harry.
Prince Harry, after his brother, Prince William, who is after his father, Prince Charles.
Currently, Prince Harry is third in the line of succession to the British throne, behind his father Prince Charles and his older brother Prince William.
Prince Charles, Prince William, Prince Harry, Prince Andrew, Princess Beatrice of York.
Prince Harry currently. It goes from Prince Charles, then Prince William then Prince Harry. Should Prince William now the Duke of Cambridge as of his marriage today if he had a child with the Duchess of Cambridge then that child would displace Prince Harry and the child of their marriage would be third inline to the British crown
Then once he and Katherine die Harry will take the throne as King.
Prince William is second in line to the throne, after his father, Prince Charles. Third in line is William's brother, Prince Henry. However, it is unlikely that Henry will become King because William's children will come before Henry in the line of succession: first the male children, oldest to youngest; followed by the female children, oldest to youngest. Unless William passes away with no children (and assuming his wife is not pregnant with the heir, in which case the heir would be king/queen and a regent would reign for him/her until he/she comes of age) the next on the throne after William will be William's eldest son, or his eldest daughter if he has no sons. See the related question link below.
If Charles dies before Queen Elizabeth dies, then Prince William will be first in line. If Charles dies while he is king (having succeeded Elizabeth to the throne), then Wlliam becomes king and his eldest son (supposing there is one) will be heir to the throne. If William does not have children, then his younger brother Harry will be first in line.
His eldest son, William. Prince Harry his 2nd son is 3rd in line to the throne.