Look for the easy things first.
Check the linkage from the shift lever to the tranny; there is a plastic busing that connects the shift rod to the tranny shift arm under the car. These can wear of break, resulting in the tranny not getting into the proper gear when shifted.
Check the vacuum line to the tranny.
check the bowden cable, or downshift cable for proper adjustment - do a search here to get nice instructions and pictures.

After that, it could get more expensive and complicated.

DG