Most of the time they are either a Judge or Lawyer,but neither is necessary as a prerequisite.The President could Nominate a Truck Driver for the Supreme Court if he wished to.
they can nominate anyone they want, but only the truly qualified can obtain the position. Sotomayor will likely be sworn in because of her outstanding qualifications. According to some, Sotomayor is "the most qualified nominee in the past 100 years".
No, but most modern presidents have always picked judges on the U.S. Court of Appeals. Presidents in the past have picked Attorney General, Solicitor Generals, Senators, etc.
There are no constitutional qualification requirements to serve on the Supreme Court, as there are for House, Senate, or President. Only requirement is confirmed by Senate. President could appoint a high school drop out. If Senate confirmed, they could serve.