Small dogs tend to get nasty teeth faster than larger dogs, because they have the same number of teeth as a large dog, crammed into a tiny mouth. It's probably too late for brushing to fix the problem. Please let your vet do a cleaning. Your dog may also need some extractions, due to decay, as Dr. Doug mentioned. BTW, dogs can eat just fine without all their teeth. I had my daughter's 7 lb. yorkie-poo for several months, and I had him get a dental right after we took him in. He had to have four teeth removed, and he was just fine.
After the cleaning, follow up with daily brushing with a toothbrush made for small dogs and a dog toothpaste. I'm partial to Nylabone toothbrushes and TropiClean Clean Teeth Gel. The gel says no brushing needed, but ignore that. Brushing is much more beneficial. I always do it at night, right before bed, so that it stays on the teeth longer. Don't allow food/water for at least 30 minutes before or after brushing for best results.