For self defense, your typically looking at a 12 gauge (the all-time favorite home defense gun). 16 and 20 gauges are also available though not as plentiful. Contrary to the numbering they are less powerful. Consequently, they kick less, and that's a definite advantage for your first shotgun! I have heard it argued that a good load in a twenty gauge makes a fine home defense gun. Someone more educated than I should fill you in on the intricacies of load, velocity, etc.
As to manufacturers, these are the top names:
(What others am I missing guys? Any lesser known names you would vouch for?)
Your second big decision is pump or auto. Personally I'd go with pump because it's a heck of a lot cheaper ... and it just sends tinglies down your spine everytime you rack it
Check this guy out: http://www.winchesterguns.com/prodin...d=252&cat=021C I haven't heard anything on it, but that's a rocking price!