Whether you intentionally bred your dog or not, you need to prepare for the birth of the puppies. This involves not just taking care of the mother but setting up adequate space in your house. Taking care of a litter of puppies isn't an easy task and you need to be prepared for it.
Take the mother to the vet and have her checked out. The vet might recommend vaccinations since she passes antibodies to the puppies through her milk. Know that gestation is 63 days long, give or take, depending on the breed. Alert your vet if the mother starts giving birth prematurely or if she's showing no signs of labor a week after her due date.
Set up a whelping box. This is usually a wooden box with low sides and sometimes a hinged "door" that acts as a ramp to the floor for when the puppies get older. Set it up in a room that is quiet and can be kept dark. Put a tarp or plastic drop cloth underneath and around the box in case of accidents. The mother will give birth in the box (hopefully) and clean her puppies there as well. Put blankets and a pillow in the box for comfort.
Encourage the pregnant female to sleep and sit in the box regularly. That helps her get used to being in there. If you install a heat lamp, that might make her more likely to go in it, and it will also help the puppies retain body heat after they're born.
Research the signs of labor. Examples include loss of appetite, restlessness and sometimes vomiting. This may happen about 12 hours before active labor. You don't have to stay with her the whole time as this can make her uneasy. At this time, quarantine any male dogs in the house, as they don't have the same instincts as the mother does and might become jealous and attack the puppies.
Check on your dog when she goes into active labor. Puppies will start to be born about an hour after she starts active labor. Keep an eye on her but don't bother her; animals know how to take care of their young instinctively. Depending on the breed, it will take several hours before the entire litter is born.