12 gauge single shot shotgun, sawed off. LOL. You can get em used for like $80. Thats a cheap home defense weapon. Just dont miss (not easy if you have eyesight and appropriate shells).
Seriously though, if you want to invest in a handgun, Glocks are great but pricey, Smith & Wesson, Berettas, etc. are all good also. You can get a gun guide at any magazine store or Barnes & Noble for like $5. It'll have descriptions, a picture of each gun, and a pricing guide.