In his first tweet President Trump tweeted:

Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism!

He next tweeted:

Is it legal for a sitting President to be "wire tapping" a race for president prior to an election? Turned down by court earlier. A NEW LOW!

The FISA Court was put in place in 1978 when Congress enacted the Foreign Intelligence Surveillance Act (FISA):

The Court sits in Washington D.C., and is composed of eleven federal district court judges who are designated by the Chief Justice of the United States.  Each judge serves for a maximum of seven years and their terms are staggered to ensure continuity on the Court.  By statute, the judges must be drawn from at least seven of the United States judicial circuits, and three of the judges must reside within 20 miles of the District of Columbia.  Judges typically sit for one week at a time, on a rotating basis.