You can start by bathing your dog with shampoo that is formulated to kill fleas. Then try treating your dog with a monthly medicine designed to prevent fleas altogether.
There are a few different methods but not all are as effective as another. For example flea baths and collars are not as good and safe as just giving them flea drops. A dog is much less likely to notice a flea drop.
Do your homework, OTC flea drops such as Hertz and Sergeants have been known to kill our furbabies! Don't believe me, do a search and you'll see a lot of grieving pet owners.