Pardon me for disagreeing, but if you are not familiar with firearms, your first choice might be a Ruger 10/22, chambered in either .22 Long Rifle or .22 Winchester Magnum rimfire. Why? You can pop off an entire magazine, with next to no recoil. Total control. With a little extra magazine capacity (available in most locations), you can literally sign your name on (actually, 'through') the perps forehead. And you can take either of these rifles to any shooting range in the country, and practice (very important), without raising an eyebrow anywhere.
There are other rifles that I would rank upfront as well, but I love the Rugers.
Second choice in my book would be a shotgun. Once again, if you're not experienced with firearms, I would suggest a 20 gauge shotgun (less recoil). Go pump (even the AK-based Saigas have reliability issues). Another option would be a 12 (or once again, a 20) with a Knoxx recoil-reducing stock (if legal where you live). I like the Winchesters (the 1200/1300 12 gauges may even feed those neat little Mexican short shells, giving you more magazine capacity ;>).
Handguns? Most folks can't hit crap with them. If you need one, go with a double-action, name-brand revolver, .38 Special or better (unless you find a nice .32 Magnum - and I still prefer .38 or better) - they're next to foolproof. Just keep pulling the trigger. Do NOT buy some elephant killing magnum, unless it also chambers a useful cartridge like .38 Special, .44 Special, or .45 Colt (in which case plan on using the lower powered cartridge for self defense). Hand guns are third choice, at best.
Just my completely unauthorized opinion. Bottom line in my book: if you can't hit the target, what good does it do...
I am not the author.