In a suburban or urban environment a 12 gauge pump is very cost effective. A combination of Number 4 buckshot & slugs is my recommended ammo. #4 buckshot is deadly but won’t over penetrate like 00 may due. Brenneke slugs are effective against cars.
Get one with a 20” barrel. Later,if you can afford it, get a 26” or 28” extra barrel and practice at the local range with inexpensive target ammo. Practice and learning safety is absolutely important. If you know someone that is a shotgunner talk to them, most are very helpful people. The gun is only as good as the operator.
For me, if you are using it for tactical/home defense purposes and not hunting you should go with the 18” barrel. The shortest that’s legal. It’s lighter and easier to maneuver in tight spaces like doorways. It also has a wider spread but inside a house the range is so close the spread will still be tight. About the diameter of a ping pong ball.