If it doesn't shift to first or second than you most likely have problems with the transmission's internal linkage, but before you assume that I would check the car's external linkage and make sure you have a good clutch. Can you shift to first and second when the car is not running? If you can not, get under the car and make sure that the linkage is not being restricted by something. If the linkage is unrestricted and you can't shift when the car is off I would say that it is safe to assume that it is time for a new trans.
I just swapped a LSD trans from a 2000 sentra into my 93 g20 and I think it is well worth the extra work, especially if you have plans of more power in the future.