When is the best time to breed my shitzu maltese?
I wish to breed my female shitzu maltese before I get her desexed as I lost my last dog of 6 years to cancer. I have never bread before so I would like as much information as possible before I start. Any help would be appreciated.

Breeding your dog isn’t going to reduce the risks of cancer. Properly spaying your dog will tho. It’s a common myth to believe that females should be allowed a litter, when in reality, doing this can be deadly to the dog. Dogs can die from birthing a litter.
The other aspect is that you have a mixed breed and breeding a mixed breed dog is entirely unethical. You would be adding puppies to a world that already has tons of puppy mills and backyard breeders putting them out like crazy. Even if you find "homes" for the pups, you could possibly be taking away the chance of a shelter dog/puppy getting rescued. If they don’t get that pup from your litter, they may go on to a shelter to rescue instead.
Please research the pros and cons of this and talk to your vet. I don’t know any vet that will tell you it’s a good idea to breed a female before spaying. The benefits to spay far out weigh the risks of having a litter.
It is best to wait until after she has her first estrus (heat). If her first heat has already come and gone, then when she comes into heat again, breed then. A dog stays in heat for about 3 weeks. It is best to breed in the second week starting on the 7th to 10th day. This is typically when she will start accepting a male. You can breed her anytime from the second week until the end of her heat.