Usually they go in to heat, not before 6 mos, depending on their size(how much does your yorkie weigh). It seems the smaller they are, the longer it takes for the first heat cycle. The first week of heat, they bleed, you will probably see her licking herself all the time, you can check to see if she's swollen(if you don't notice any blood). Please, really read up on pregnancy, whelping, etc before breeding. I'm going to start breeding, but, not for over a year yet, and I have a lot of reading, etc to do. There is so much that can go wrong with these little ones when breeding, like losing the female, or losing puppies, or both. Sometimes the bitch has to have a c-section(which can be very costly). There have been breeders who have lost the bitch while whelping and even afterwards, and the breeder has had to bottle feed every baby for the first 3 weeks or so, every 1 1/2 to 2 hours. These are all things you need to be prepared for. You will also need to get your female in to your vet and have her checked out good and tested to make sure she is okay to breed and doesn't have any hereditary things she could pass on to her babies.
Oh by the way, LOL, she is gorgeous. And now a question, I can't tell by the picture, but does she have a collar/leash on or harness? If collar, please change to harness, they are much more safer and having a collar on them can mess up their tracheas. You may have a harness and it's just hard to tell with all the hair.
