You should hire an electrician to help you with this, since you don't sound qualified to wire up a generator properly. She can help you figure out the right way to do this. However, it's good for you to be well-informed going in to this. So:
Generators are rated in several ways:
Peak load. Starting a motor (like a pump or fridge compressor) draws a lot of current (amps) for just a moment. You want to make sure your generator can handle that.
Nominal load. If your fridge and sump pump and well pump are all running at the same time, can the generator handle the load? Add up all the loads of what you plan to run. I've ready that it's good to keep a generator running at 50% - 75% capacity most of the time, but that was in an off-grid RV context; it may or may not apply to you.
240V vs. 120V. Smaller generators usually only put out 120V. If your sump pump or well pump requires 240V, you'll need one that can do that.
Fuel type. If you already use propane or natural gas for heat, it can be convenient to use that fuel for your generator. Otherwise you'll need to keep gas or diesel on hand. Diesel is less explosive. Gas engines are easier to work on. Use a fuel preservative like Sta-Bil to keep the gas fresh.
Be sure to exercise your generator regularly, per the manufacturer's instructions. (My RV generator wants to run for 30 minutes at 50% load every month.)
A permanently-installed generator must be on a transfer switch. This will disable the connection to the power grid when the generator is providing power. This is critical to protect line workers doing repairs on the grid after a storm. You can even have the whole system be automatic, so if you are evacuated or on vacation the generator can kick in and save your stuff.
Think about where you want the generator. Close to the transfer switch & main breaker panel will reduce cost of wiring.
The smaller your generator is, the cheaper it will be to buy, install, maintain, and run. A simple way to approach this problem is to buy a portable generator that will run one of those 3 loads at a time (sump pump, well pump, fridge). Plug each appliance in to the generator one at a time, as needed. Pump out the basement, then fill the well reservoir, then draw down the fridge. (Filling your fridge with bottles of water will add thermal mass.) Remember that the generator must remain outdoors, to avoid carbon monoxide poisoning. This only works if your pumps are plug-in, not hard wired, and if they're close enough to run a reasonable extension cord to them from the generator.
