Congrats on the SuperNova!!
18" shotguns aren't uncommon at the ranges, don't worry.
Bring your wife with you, you'll have a great time, it'll be a lot more fun with her.
You'll want to start with some 2-3/4" shells, either "target" or "low-recoil" shells. You can buy these just about anywhere, even Wal-Mart. After you become proficient using the "light loads" you can work up to the heavier stuff. The main thing that you are trying to do is to make sure your wife is comfortable shooting the weapon, practice and train, that way if there is ever a situation that arrises, she won't be "affraid" to use the gun and therfore more likely to hit the intended target.
Unless the gun you bought is a "turkey" model the rear sights probably aren't adjustable, you don't want to "AIM" the shotgun anyway, you want to "POINT" it. Since you have a CYLINDER BORE with no choke tube so you will want to practice shooting at targets in the 15 - 25 yard range. Check with the range, tell them you want to practice on stationary targets first and they will put you on the right range.
Good luck with your new shotgun, enjoy and let us know how you and your wife do.