Sounds as though the gear linkage needs adjusting, very easy to do, haven't done one in ages, but basically it is put into a certain gear, I think it is 5th on a 5 speed and 2nd on a 4 speed, then a pin inserted through hole in the linkage, then tightened up on the clamp, a haynes manual would be a good bet, as my memory isn't!!
tabetha