The first step is to add a sub-panel to your existing electrical box. There are sub-panels that are specific to generator use that isolate the sub-panel from the main panel. Move your "essential loads" from the main panel to the sub-panel. You will also need to install a generator hook-up outside your home as close as possible to the sub/main panel.
Generators are marketed by their wattage. In order to properly size the generator you need to determine the consumption of all of the loads that will be powered by the generator and make sure that generator has the ability to provide the continuous power requirements as well as a 50% allowance for in-rush.
Refrigerator 10a (10a x 110v = 1100w)
Microwave 5a (5a x 110v = 550w)
Boiler 10a (10a x 110v = 1100w)
Well Pump 15a (15a x 220v = 3300w)
Livingroom Circuit 10a (10a x 110v = 1100w)
Total power required is 7,150w continuous with the ability to deliver a demand of 10,725w.
Ten years ago I purchased such a generator as I grew tired of the frequent power outages in my area. In the ten years that I have had this very capable system I've yet to use it. It would appear that the local power company made improvements to the power delivery at the same time I purchased a generator, outside hook-up and sub-panel. But I'm ready.