The President stated his offices were wired for spying, during his campaign. While many in the media, and in politics, are claiming Trump is lying, I doubt he'd makes such a bold claim without some evidence.
Supposedly, a FISA judge authorized the surveillance in October for national security purposes. The reason, as reported, was for national security, which, since it wasn't a criminal investigation, gave the Obama Administration access. The question is what information, if any, was gathered, who accessed the information, and who instigated the request for surveillance.
I doubt former Presidents didn't do such things; and we know Nixon was culpable in the attempt to bug the Democratic National Committee headquarters at the Watergate Hotel. The President has broad powers, and can use many without public scrutiny. Abusing the power frightens citizens, since the power can lead to violence against those that are perceived as threats.
Where this all goes is a crap shoot, but the underground bureaucratic part of our government is obviously out of control. Trump exposing it is a step in the right direction; but I have the feeling the backlash by this shadow government will soon be seen. Operating on the fringes, or outside the bounds of legal standards leads to a secretive, sometimes dangerous, branch that doesn't answer to those that fund their endeavors.