You're right, Sarah....this is a wide open area of discussion.
There are a range of issues you'll need to narrow down as you go - manufacturer, pump or semi-auto, gauge, size of the gun's stock and its length of pull (are you short or tall ?), and what kind of ammunition you will stoke the gun with for Home Defense.
First of all, Remington and Mossberg are far and away the most popular Home Defense shotgun makers. The Remington 870 is the "Coke" of the HD shotgun world and the Mossberg 500/590 is the "Pepsi". Both are reliable, and will do the job you need. They are very similar - the primary difference being the location of the safety.
Other, less pricy guns you could consider are Charles Daly, and Maverick (made by Mossberg).
Generally, semi-automatic shotguns offer less recoil, but may have trouble cycling some loads. Semi's are also more finicky about maintenance and cleaning. Pump guns, on the other hand, will push you a little more but will usually run any ammunition you put in it.
The size of your gun's stock and it's length of pull are crucial. If your gun doesn't fit you, it will hurt you and you won't practice with it. If you're shorter of stature, you may want to consider youth stocks.
Both Remington and Mossberg make 20 gauge, downsized shotguns for youth and small female shooters. These are capable of performing well in a Home Defense role.
As far as ammunition.........? Much of that depends on if you live on a farm or in an apartment building. 00 buck is considered the "gold standard" of 12 gauge HD ammunition, but again there are a tremendous amount of variables, and this is an area where there is less than a unanimous opinion.
This should get you started. If you're near Middle Tennessee, we're offering a class called General Defensive Shotgun on April 19. That might be of interest to you.
Hope this helps............good luck.