Most likely is the linkage is out of adjustment, worn, bent, or has a bad bushing. Before doing any work on it, make sure the wheels are chocked (or ideally, the car is up in the air on stands). If all the bushings and connections are good and tight, disconnect the shift linkage right at the first lever on the transmission. Push that lever all the way to the rear (that's Park). Check to verify that the driveshaft won't turn. Then put the shifter in the Park position. Adjust the linkage so the last rod will slide into the hole in the lever without having to force it. Move the shifter to 1st/low. Pull the link back off the lever and verify the trans in in low (the forward most position) There again the link should slip into the lever hole without having to move the linkage or forcing it in. Sometimes adding a washer to various linkage connections will help tighten things up if you have a lot of slop. Just make sure nothing binds and that the hairpins/cotter pins are installed and in good shape.