There is no King at the present time. Queen Elizabeth had no brothers, and just one sister who was younger. Therefore, when her father died (he was the King profiled in the recent film, "The King's Speech" starring Colin Firth), she inherited the throne as the oldest child. (While her father's wife, also named Elizabeth, was called Queen, she was not in the royal line and did not inherit when he died. She became known as the Queen Mother at that time. When Elizabeth the daughter (current queen) married, her husband Philip was given a series of honorary titles, including Prince Philip and the Duke of Edinburgh and many others. I believe he got some when they married, and possibly some additional ones when she ascended to the throne. He cannot become king as he is not in the royal line.
Since Elizabeth had 4 children, they were in line for the throne in birth order for the males - Charles is the oldest, so the throne was to be his. After him, originally it was his 3 siblings: Andrew, Edward and Princess Anne (she is last because she's a female!). However, once Charles had children, they come next in the line of succession - William is the oldest so he's next.
Kate can never become queen. She will probably be given a royal title - probably Princess and most likely some additional titles. That's what was done with Diana.
So, the current line of succession is:
Charles
His son, William
His 2nd son, Harry (until William and Kate have a child, then that child is next after William and before Harry)
Prince Andrew (Charles' brother)
Princess Beatrice (Andrew's eldest)
Princess Eugenie (Andrew's 2nd)
Prince Edward (Charles & Andrew's younger brother)
Prince Edward's children (not sure if he has any yet but I think there's a baby)
Princess Anne
Princess Anne's kids, starting with her son.
Everyone would have to either die or abdicate for this to occur.
Does this help? or make it more confusing???