This can be caused by a number of things, Easy stuff first.
Check and bleed the clutch fluid (it is a hydraulic system )
The clutch pedal cross shaft ( the brake pedal pivots on the same shaft) has nylon bushings that wear out... If they're gone altogether it can introduce enuff play in the system that the clutch won't completely disengage.
The pilot bearing being worn out can cause this, as can the input shaft bearing ( both of these involve pulling the transmission, sorry)
I've heard of cracking in the firewall area around the master cylinder, but I haven't seen this myself yet.
A quick way to get it into gear when it's like this is grind the sucker into reverse first, this will stop the input shaft from spinning, then you should be able to get it into first easily.