How do I fill a gap at the bottom of a door?

The back door to my house has a small gap at the bottom on the side nearest the hinges. I think this could be letting bugs into my house as well as raising my power bill, but I'm a girl without a large amount of tools or knowledge of home improvement. How can I easily fill this gap? Any solution that might involve household materials I'd already have around would be especially helpful.

Depending on how large the gap is, you might be able to use some stick-on foam insulation strips. They are available in home centers (Lowe's, Home Depot), and you could put one layer over another if needed.

Sorry, not sure of any household items to make this repair, but the "tack on" weatherstrip you can get at any hardware store is not real hard to apply.
If you decide to go this route measure your door before you go to the store in case they carry different lengths.
A hammer is the only tool required, and maybe a knife to cut of excess.
You can also get a type that has an adhesive to attach it but I don't particularly like it as it seems to fall off very easily.

You can get a door shoe, which is a metal strip with a rubber seal attach. You can pick it up at your local home center, and it comes with all the direction.

It has to be cut to length, and slips over the bottom of the door. Or there is another one that you screw on the face of the door.

Be sure to ask the sale person for weather stripping for you exterior door.

There called door sweeps you can get at almost any hardware store

You can buy those gap fillers at ANY hardware store