An individual from any breed can be a service dog. It depends on the tasks necessary. A yorkie wouldn't be able to be a guide dog or a mobility dog but could be an alert dog or possibly a PSD.
To have a service dog, three things must be true:
1) The handler MUST be disabled per the ADA. This means that they must have a physical, medical or psychiatric condition that significantly negatively impacts one or more major life activity such as walking, thinking, learning, caring for oneself, etc...
2) The dog must be trained to mitigate the handlers disability. This means that the dog must do something that the handler can not do for themselves due to their disability. Providing comfort or making one feel better just by being there does not count.
3) The dog must be trained to behave properly in public. A very small percentage of dogs have the proper temperament for service dog work. They must be able to ignore sudden, loud noises without reacting, ignore people trying to get their attention, ignore food, not sniff shelves, not seek attention, stay focused on the handler at all times.
It takes 18 mo - 2 years to train a service dog. While not legally required, if you don't know what you are doing (and by your question, you don't), you need to hire a trainer to be successful. Training is NOT cheap...no where near. It cost $20K-$50K to fully train a service dog, and an owner trainer would have to cover all of the cost.