Power in to junction box:

If you have power going in to the junction box then they are using a 14/2 or 12/2 wire going from the junction to the switch box with one of the wires re-purposing the neutral as a switch-leg. In which case, both neutral and black are HOT. In this circumstance you should see a neutral marked as black in the switch box and junction box (not all electricians do this). If you have two blacks then your wire must have been run in conduit (super easy to add a neutral then).

In this scenario you can just abandon the current wires going to the switch and wing nut them off and mark them in the switch and junction box. Then put a new switch box next to the junction box using 14/3 between them. The wiring would be the same as in the diagram above however the white wire would be switched with a red wire (switch device would have 1 black and 1 red attached) and neutral would stay neutral in your new switch box.

OR do the same wiring scheme that I just mentioned but abandon the 14/2 (again) and run new 14/3 to the current switch box from the junction box.

OR if your wires are in conduit, fish a neutral to the junction box through it and you don't have to do any other work but connect it to the bundle in the junction.

Option b is that the power goes in to the light box, in which case you would have a neutral in there; but I take it that is obviously not the case.

If you have any question feel free to ask, but do include pictures of your junction and switch box wiring.

** Please note: if this is a 20 amp circuit (which I doubt) you would need 12 gauge wire. You can tell if it is by looking at your panel, the romex jacket, or just judging the thickness; 12 gauge wire is harder to bend.