It's important to know how to use a generator if it's going to be a primary source of energy on an RV or sailboat or if you'll need it to backup your primary energy source. Knowing how to use a generator before it comes time to need to know how to use it can save you a lot of trouble.
Generators should be used outside or in an otherwise open area (in other words, not your garage). Never use a generator in an enclosed space; generators produce carbon monoxide, which is an odorless, colorless gas and can be deadly. Because of that, you also want to avoid keeping the generator near an open window or door and make sure the exhaust points away from the house. It can be impossible to detect carbon monoxide in your home and you may not realize you have been poisoned until it's too late.
You also want to keep your generator dry to avoid electrocution. Keep it under a canopy if it's raining or snowing and make sure you don't leave generator extension cords sitting on the wet ground.
Make sure your generator is ready to go before an emergency. This means making sure it's in functional condition and is oiled up. Make sure the generator is completely cool before adding fuel. You'll need an extension cord to plug the items you wish to use into the generator. There are extension cords made specifically for generator use; these are ideal to use.
Make sure you don't overload your generator. Before use, determine how many watts your generator can handle at once and don't go over that. To determine how many watts your generator will need to be able to handle, add up the wattage of all appliances or items you plan on using. Your generator should be able to handle this number, at a minimum.
Don't try to power your whole house using a generator; you can electrocute nearby utility workers. If you want to use your generator to supply energy to your whole house, have a licensed electrician install a power transfer switch in your home before you need to use the generator.
What size generator do I need? This largely depends on your intended use of the generator. Whether you'll be using the generator as a backup source of energy for your house, as the main source of energy for an RV or a live-on sailboat or in case of an emergency for a large building such as a nursing home, your needs will greatly vary.