If it's just a ding, the earlier post about dampening the spot and hitting it with the heat iron is right on.
If that won't get it out, use some Lightweight spackle-available at any hardware store. Two bucks for an essentially "lifetime" supply (pint).

Of course, a real hole has to be repaired first, than it can be smoothed and sanded flush using a litttle spackle to fill the imperfections.

Obviously all sanding should be done with a proper sanding block. The spackle sands more easily than balsa and can be feathered to a very thin edge. It works better to sand the spackle in one direction only. If you go back and forth, sometimes the spackle dust will "pill" just like eraser crumbs.

If the spackle dries out, you can add a few drops of water to the container and stir it. When you first put the spackle on the wing, it seems to help to massage it into the spot. Fingertips are good for this-Spackle washes off easily. After the spackle is worked a bit, it liquifies and attaches to the wing surface much better than when it first touches the wing.