The problem probably isn't the transmission. The clutch is not properly disengaging, or at least the transmission input shaft is not being disconnected from the engine.
If you start the engine with the transmission in gear and the clutch pedal depressed, does the car lurch foreword? If it does, the clutch is REALLY not releasing. If the car will start in gear with the clutch depressed, but it just won't shift into gear after being in neutral, it may just be dragging. Let's start with the clutch not releasing.
First, check that the clutch cable retaining e-clip (in front of the firewall) is in place, and that the release fork is being pulled forward in the bell-housing when the pedal is depressed (that is, the cable is functioning)
If the cable is OK, it could be that the friction disk has become rusted to the flywheel/pressure plate (which often happens if the car has been sitting in a damp location for a time). Sometimes you can "break" it free (sometimes figuratively, sometimes literally) by jolting the drive-train with the clutch depressed. My favourite way? Have four of your strongest friends lift the rear end so that the wheels can spin, start the engine while in fourth gear, rev the engine a bit, depress the clutch, and (while pointing the car in a safe direction!) drop the rear wheels. Don't try this while pointing the car at your home!
Other reasons the clutch is not disengaging? The pilot bearing may be piled up (which can also cause the input shaft to drag), or the release forks on the pressure plate are worn away.
If it is just a "dragging" problem, it can be a blown disk (they separate if the rivets are worn through), the aforementioned pilot bearing is damaged, the driven disk is sticking to the transmission input shaft and not "centralizing" when it is released, or the clutch is simply out of adjustment. Read the following thread:
HTH and let us know what you find.