I bought the car a month ago and it was suppose to have a new transmission in it. I put the car in drive and it did not want to move. It even started to roll back. what could be wrong with it.

Well depending , the shifter may be improperly adjusted after the transmission was installed. Wut yu can do is while shifting to select drive. When in the "N" position , slowly pull the shifter down to a position between the N and D indicator to see if the transmission engages into drive. Yu can slowly vary its position between the N and D , this will sort of help give an idea if the shifter cable is not correctly adjusted. Its a posibility since the transmission was replaced.