I clip my dogs' two front paws every week. The back paws are rarely cut as they seem to stay short on their own. I'm obsessive with nails but generally, people cut them every 2-4 weeks.

To clip them, you clip the white part of the nail (unless your dog has black nails). The pink part is the quick and it's something you definately don't want to cut. It will cause some pain and make the experience much worse for the dog. Just be sure to clip slowly with little bits at a time so you don't accidentally cut the quick.

I use dog nail clippers but some people also dremel their dog's nails.

It's probably best to have a groomer, vet or a dog experienced person to show you first just until you understand how to do it.