House training is a difficult process with all dogs, but can be especially trying with small breeds.
Due to their tiny bladders, small dogs are often unable to "hold it" and have frequent accidents.
There are several things you can do to help eliminate accidents in the house.
First, take your dog outside as often as possible! This is often the simplest solution.

• Whenever you're home, take your dog out every 1-2 hours.
  Praise her/him every time they eliminate outside.
  Immediate praise is necessary.
  An excited "Good girl!" and some petting should be sufficient.
  Your dog will eventually associate toileting outside with positive reinforcement.
  This will make her/him more likely to eliminate outside and not on your carpet!
• When you're not at home, you can attempt to crate the dog, but if you're gone for a long period of time they will likely soil their bedding.
  An alternative could be confining the dog to a tiled area (like the bathroom) and putting down some newspapers or a puppy pad.
  That way if s/he does go, it will be easily cleaned up.
  If the dog has an accident while you are gone, scolding when you come home will do no good.
  S/he will not be able to associate the scolding with the 'accident' if a long period of time elapses between the two.
  (Remember, toileting on the paper or pad is NOT an accident, you want them to go there! )

If you see the dog toileting in the house, not on the newspaper or puppy pad, just sternly tell them "No," and take them outside immediately.
If you stick to this routine, accidents in the home should be almost completely eliminated!