My Yorkie Is Losing Her Hair Really Bad And She Is Scratching All The Time But She Doesn't Have Any Fleas,her Hair Fills Brittle Too.what Should I Do?

Pruritis (itchy skin) can be caused by infections (bacteria, yeasts), parasites (demodectic mange, sarcoptic mange, fleas, other mites), or allergies.  In order to get to the bottom of your pet's problems you will have to go to the vet.  They will gather a good history of the problem, do a physical exam and some simple skin tests.  Depending on the results it might be necessary to do further tests.  Finding an underlying cause is often a process of elimination and can take some time to work out, however, your pet will only be able to get relief if the underlying cause is diagnosed and treated appropriately.  

I have a yorkie too, and they often have very dry skin. A medicated shampoo maybe once a month may help, not too often though, bathing will aggravate dry skin on dogs.  Also it is possible she has allergies, in which case your vet can prescribe meds.