If you have no previous firearms experience i suggest that you go to the nearest range/training facility near you and see if they offer intro classes on introduction to gun safety safety. The NRA has certified instructors through out the country. The other option is going to a training facility like Gunsite.
I have a remington 870 police magnum. I kept the barrel length at 18 1/2. For defensive loads i use Federal Powershok f127rs (rifled slugs). There are a few things that need to be cleared up first is you never, ever use a bird shot as a defensive round the pattern is way to large even at 15 yards.
WMD and deuce mentioned innocent bystanders gettin hit or rounds going threw walls. Deuce wrote: You do not have to be a good aim and the sound of the shell being chambered is very distinctive and can deter many a people. Mike, You shoot a shotgun like you shoot a rifle meaning sight alignment and good trigger control, none of the "pray and spray" stuff that is being said. You factor the proper fundamentals of proper gun safety,trigger control,sight alignment and lots and lots of rounds downrange the "what if scenario" wont happen. Mike if you need more help let me know.
BTW deuce also wrote: With a semi-auto you will have to change the clips every 6 months to a year as they tend to loose their 'springyness'. Funny my Glock 21 "clips" magazines have been with me for 3 years and i havent had one