It would be helpful to know more about the hole, because different damage needs different kinds of fixing.
Though when fabric wears out, you can't do all that much, to extend the life of your jeans (especially if they're $100 jeans or part of a limited wardrobe) you can fix most holes.
A simple rip in a straight line, like a broken seam or a tear, can be trimmed to remove the extra fuzz and stitched back together. A zigzag stitch on a machine is one option, but you can also do it by hand. if you've never hand stitched, what you're doing is basically lacing the gap back together.
A small hole can be mended with a piece of denim fabric (or other rough material) cut to fit and then stitched over the hole. For a funky look, shapes (skulls, hearts, etc...) make good patches. Otherwise find as close a fabric match as you can. Either way make sure that the edges of your patch are firmly stitched down to prevent it from peeling or fraying.
If you want, you can hem the patch before you sew it down. Use an iron to fold the hem over for nice crisp edges.
For a bigger hole, you can lay a patch over it. To make the item long wearing, say for the knee of a pair of pants, put a patch on the inside as well. A comfortable, non-chafing fabric goes on the inside, with denim or a rough fabric on the outside, and the worn jeans in the middle of the sandwich.