I have 2001 f150 4.2 v6.when I put it in reverse it goes forward,I need to know what could be the problem

 

Sounds like you need to adjust the linkage.

 

assuming the linkage is adjusted correctly.. the only other time I have seen this happen the one way mechanical diode clutch had seized and the intermediate clutches welded themselves together. This resulted in the vehicle going forward with the shifter in Reverse or Neutral and the wheels would lock up with the shifter in the Drive/Forward selection.