You should get a Naso Tang... LOL Just kidding.
In all honesty, there are not many fish that are recommended for a tank this size. For example. Cardinals, Dottybacks and Royalgammas are all recommended for 20 gallons and above. You could get a small False Percula Clown or a Green Chromis, which I think can live fine in a 12 gallon. However, please note that you should not have more than those two.
If you do get more, they will eventually fight for resources and your nutrient load will be much higher, which in turn will bring other issues like GHA and other things of that nature.
Good luck with your new tank and hopefully you will find something nice to add to the tank.