Wont change gear

But the guy i bought the car from said he just replace the trasmission..

Check your shift cables. If they are broke or hung this will happen. Will your shifter move freely? If so check the trans linkage. There is a pin thru the linkage and shaft out of trans. If this pins slides out some the shifter will move but the gears won't change. If this the case push the pin back thru the shaft and linkage. I use a large zip tie around the shaft to hold the pin in the shaft. If shifter will not move then remove cable ends from the trans shift linkage then have someone move the shifter while you watch the cable ends. If they move then they are probably ok. i would then look into the potential of trans issue.