Polishing teeth to make them shiny?

Go to a dentist for help unless you want to risk ruining your teeth while attempting to perform procedures which could likely damage your teeth.

Yes, a dentist would definitely be able to smooth this for you. It is not a good idea to use sand paper to file your teeth and I would refrain from doing that ever again. Always visit a dental professional with concerns of your teeth.

Yes, we have various different polishers available to make your tooth smooth again. The dentist can also smooth the corner for you as well. 

I would suggest in the future not using sand paper on your teeth. It really takes very little for you to do irreversible damage. Your dentist may be able to polish your tooth to bring back the natural luster. It will depend on how deep the scratches are.