All that's coming out is clearness.

Production of milk varies greatly between dogs, in my experience it won't happen until the very late stages, around week 8 (1 week before whelping) some even later.

How far along is she?
If she is very near her due date you may have to buy some milk replacement formula from your local pet store or vets and bottle or tube feed the puppies.
Bottle feeding is relatively easy, but tube feeding you will need to consult your vet about.
It is not something I would attempt just be reading about, you need someone to physically show you how to do so. Tube feeding involves running a tube directly into the puppies stomach. Sounds frightening I know, but it is only a last resort, most pups do take well to the bottle and it is not needed.

Keep an eye on the clear fluid and keep an eye on the condition of the teats.
If the fluid changes color and becomes yellow or green this is an indication of infection and will need to be seen to by a vet. Infection of the milk glands is called mastitis if you want to do a bit of research.

Do you have everything prepared for whelping?
If you need to ask any questions about the process feel free, we have several knowledgeable dog experts on this site who can help guide you through the pregnancy and whelping.