I have to tap the a.c. compressure to get 1995 nissan altima to start sometimes why?

Is the car hard to start or the AC?? if you hitting the compressor to get it to start, the clutch maybe worn out or is coming loose, check the nut in the center of the clutch and see if it is tight,
if the engine is hard to start, fallow the directions that Kenneth White gave you! Note: if you have to hit the starter to make it work, it needs to be replaced.

Sounds like it is starting to lock up. Take the accessary belt off and try to turn the compressor by hand and see if it feels rough. If not, try the starter, if it still wont crank , hit the starter lightly with a hammer. Then if it starts, thats the problem. By hitting the ac compressor you may have been jolting it enough that it shook the starter enough to make contact.