very basic confusion

[Akash aki]

No: HPFGUIDX 163655

hi
  May be it was discussed earlier, I am not aware of it so no offence.
  I was thinking and got confused how it all got started, that Potters realised that Tom Riddle is behind their son. Prophecy was something ' he will mark other to his equal' but why he chosen Potters' son.
  Also how DD and others so convinced that it is going to be HP whom Tom is going to mark and which led to all this FC, SK etc, and how Tom decided about Potters's son. 
   
  aki