first of all, get a 12 guage.
second, dont get a single shot!
third, load up with slugs or 00buckshot for HD.
now with shotguns, being able to hold more is ususally better cuz your not gonna be reloading quickly unless you get a saiga. so I suggest a mossberg 590 (8+1 in the chamber=great!). cost about 350 from big5 when its on sale. its better then an 870 cuz reloading and getting your finger stuck in the mag area sucks!!!! plus it holds more and is cheaper and better.
now that you got your shotgun, go to a range and start off with birdshot and move your way up. 12guage does kick but if you learn to shoot it right, it will put down a bear if need be. if the recoil is too much, get a limbsaver as well as a knoxx spec ops stock to tame it (it does work if you use it correctly).